feat: let the viewer pick the audio track

The streaming path mapped 0:a:0 unconditionally, so a dubbed film played in
whichever language was muxed first and the others were unreachable. The node
now enumerates the tracks in stream_init and honours audio_track in
stream_req; switching is the seek path, since one ffmpeg carries one track.

MNP 3.2, additive: the player draws its selector from the node's own list and
never from a version number, so an older node is never asked for a track it
would ignore. MNP_MIN_SUPPORTED does not move.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

13 files changed, 362 insertions, 6 deletions

diff --git a/packages/meshbay-hub/src/meshbay_hub/static/locales/de.js b/packages/meshbay-hub/src/meshbay_hub/static/locales/de.js
index cb92b18..806fa38 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/de.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/de.js
@@ -249,6 +249,8 @@ export default {
   'video.buffering': 'Wird gepuffert …',
   'video.resumed_at': "Fortgesetzt bei {time}",
   'video.from_start': "Von vorn beginnen",
+  'video.audio_track': 'Tonspur',
+  'video.audio_track_n': 'Spur {n}',
   'video.close': 'Schließen (Esc)',
   'preview.pdf_fallback': 'Dieser Browser zeigt das PDF nicht direkt an. Laden Sie es '
     + 'stattdessen herunter — entschlüsselt wurde es ohnehin hier.',
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/locales/en.js b/packages/meshbay-hub/src/meshbay_hub/static/locales/en.js
index 3b84c3c..92114c9 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/en.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/en.js
@@ -248,6 +248,8 @@ export default {
   'video.buffering': 'Buffering...',
   'video.resumed_at': "Resumed at {time}",
   'video.from_start': "Start from the beginning",
+  'video.audio_track': 'Audio track',
+  'video.audio_track_n': 'Track {n}',
   'video.close': 'Close (Esc)',
   'preview.pdf_fallback': 'This browser will not display the PDF inline. Download it instead — it was decrypted here either way.',
   'preview.too_large': 'This file is {size}, more than this page can hold in memory ({limit}). Download it instead — a download is written straight to disk.',
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/locales/es.js b/packages/meshbay-hub/src/meshbay_hub/static/locales/es.js
index 145f464..d9b8f63 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/es.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/es.js
@@ -247,6 +247,8 @@ export default {
   'video.buffering': 'Almacenando en búfer...',
   'video.resumed_at': "Reanudado en {time}",
   'video.from_start': "Empezar desde el principio",
+  'video.audio_track': 'Pista de audio',
+  'video.audio_track_n': 'Pista {n}',
   'video.close': 'Cerrar (Esc)',
   'preview.pdf_fallback': 'Este navegador no mostrará el PDF integrado. Descárguelo '
     + 'en su lugar — en cualquier caso se descifró aquí.',
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/locales/fr.js b/packages/meshbay-hub/src/meshbay_hub/static/locales/fr.js
index a4f32de..d507d28 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/fr.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/fr.js
@@ -248,6 +248,8 @@ export default {
   'video.buffering': 'Mise en mémoire tampon...',
   'video.resumed_at': "Reprise à {time}",
   'video.from_start': "Reprendre depuis le début",
+  'video.audio_track': 'Piste audio',
+  'video.audio_track_n': 'Piste {n}',
   'video.close': 'Fermer (Échap)',
   'preview.pdf_fallback': 'Ce navigateur n’affichera pas le PDF directement. '
     + 'Téléchargez-le plutôt — il a été déchiffré ici dans les deux cas.',
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/locales/it.js b/packages/meshbay-hub/src/meshbay_hub/static/locales/it.js
index 1da0793..b6c68dd 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/it.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/it.js
@@ -248,6 +248,8 @@ export default {
   'video.buffering': 'Buffering in corso...',
   'video.resumed_at': "Ripreso da {time}",
   'video.from_start': "Riparti dall'inizio",
+  'video.audio_track': 'Traccia audio',
+  'video.audio_track_n': 'Traccia {n}',
   'video.close': 'Chiudi (Esc)',
   'preview.pdf_fallback': 'Questo browser non mostrerà il PDF nella pagina. Lo scarichi '
     + 'invece — in ogni caso è stato decifrato qui.',
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/locales/ja.js b/packages/meshbay-hub/src/meshbay_hub/static/locales/ja.js
index 971ecb2..99e2e1f 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/ja.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/ja.js
@@ -245,6 +245,8 @@ export default {
   'video.buffering': 'バッファリング中…',
   'video.resumed_at': "{time} から再開しました",
   'video.from_start': "最初から再生する",
+  'video.audio_track': '音声トラック',
+  'video.audio_track_n': 'トラック {n}',
   'video.close': '閉じる（Esc）',
   'preview.pdf_fallback': 'このブラウザーはページ内に PDF を表示しません。'
     + 'ダウンロードしてご覧ください。いずれにせよ復号はここで行われています。',
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/locales/nl.js b/packages/meshbay-hub/src/meshbay_hub/static/locales/nl.js
index dae63d2..269fa70 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/nl.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/nl.js
@@ -249,6 +249,8 @@ export default {
   'video.buffering': 'Bezig met bufferen...',
   'video.resumed_at': "Hervat op {time}",
   'video.from_start': "Vanaf het begin afspelen",
+  'video.audio_track': 'Audiospoor',
+  'video.audio_track_n': 'Spoor {n}',
   'video.close': 'Sluiten (Esc)',
   'preview.pdf_fallback': 'Deze browser toont de PDF niet in de pagina zelf. Download '
     + 'hem in plaats daarvan — ontsleuteld werd hij hoe dan ook hier.',
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/locales/pl.js b/packages/meshbay-hub/src/meshbay_hub/static/locales/pl.js
index 639d518..363296c 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/pl.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/pl.js
@@ -254,6 +254,8 @@ export default {
   'video.buffering': 'Buforowanie...',
   'video.resumed_at': "Wznowiono od {time}",
   'video.from_start': "Odtwórz od początku",
+  'video.audio_track': 'Ścieżka dźwiękowa',
+  'video.audio_track_n': 'Ścieżka {n}',
   'video.close': 'Zamknij (Esc)',
   'preview.pdf_fallback': 'Ta przeglądarka nie wyświetli pliku PDF na stronie. Proszę '
     + 'go pobrać — i tak został odszyfrowany tutaj.',
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/locales/pt-BR.js b/packages/meshbay-hub/src/meshbay_hub/static/locales/pt-BR.js
index 371b84f..2a90956 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/pt-BR.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/pt-BR.js
@@ -249,6 +249,8 @@ export default {
   'video.buffering': 'Armazenando em buffer...',
   'video.resumed_at': "Retomado em {time}",
   'video.from_start': "Começar do início",
+  'video.audio_track': 'Faixa de áudio',
+  'video.audio_track_n': 'Faixa {n}',
   'video.close': 'Fechar (Esc)',
   'preview.pdf_fallback': 'Este navegador não exibirá o PDF na própria página. Baixe '
     + 'o arquivo — de todo modo ele foi descriptografado aqui.',
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/locales/zh-CN.js b/packages/meshbay-hub/src/meshbay_hub/static/locales/zh-CN.js
index 96ba4fe..e3aa440 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/zh-CN.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/zh-CN.js
@@ -243,6 +243,8 @@ export default {
   'video.buffering': '正在缓冲…',
   'video.resumed_at': "已从 {time} 继续播放",
   'video.from_start': "从头开始播放",
+  'video.audio_track': '音轨',
+  'video.audio_track_n': '音轨 {n}',
   'video.close': '关闭（Esc）',
   'preview.pdf_fallback': '此浏览器不会在页面内显示该 PDF。请改为下载——无论如何它都已在本地解密。',
   'preview.too_large': '该文件为 {size}，超出本页面可在内存中保存的上限（{limit}）。请改为下载——下载会直接写入磁盘。',
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/transport.js b/packages/meshbay-hub/src/meshbay_hub/static/transport.js
index a6b3f52..262b00e 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/transport.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/transport.js
@@ -2264,12 +2264,24 @@ class MeshBayTransport {
    * MediaSource consumes a segment at a time — which is fine for a clip and
    * fatal for anything worth streaming.
    */
-  requestStream(fileId, credits = STREAM_CREDITS, start = 0) {
+  requestStream(fileId, credits = STREAM_CREDITS, start = 0, audioTrack = null) {
     // `start` is a seek: the node retires whatever this session was streaming
     // and spawns ffmpeg again from there. Omitted or zero is the film's
     // beginning, which is what an 0.1 node understands.
-    console.log('[stream] sending stream_req start:', start, 'credits:', credits);
-    this._send({ type: 'stream_req', v: '0.1', file_id: fileId, credits, start });
+    //
+    // `audioTrack` is omitted entirely unless the caller has one, and the
+    // caller only has one because a `stream_init` listed the tracks. A node
+    // too old to enumerate them is therefore never sent a field it would
+    // ignore — which matters more here than it looks: ignoring it would not
+    // degrade the stream, it would serve a different language without saying
+    // so.
+    const req = { type: 'stream_req', v: '0.1', file_id: fileId, credits, start };
+    if (Number.isInteger(audioTrack) && audioTrack >= 0) {
+      req.audio_track = audioTrack;
+    }
+    console.log('[stream] sending stream_req start:', start, 'credits:', credits,
+                'audio_track:', req.audio_track ?? '-');
+    this._send(req);
   }
 
   /** Room for `n` more segments. */
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/video-player.js b/packages/meshbay-hub/src/meshbay_hub/static/video-player.js
index 4f8a4e4..4970262 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/video-player.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/video-player.js
@@ -1,7 +1,7 @@
 import {
   html, useState, useEffect, useCallback, useRef,
 } from './vendor/htm-preact.js';
-import { t } from './i18n.js';
+import { t, getLocale } from './i18n.js';
 import { Icon } from './icon.js';
 import { formatSize } from './file-utils.js';
 import { loadAuth } from './hub-client.js';
@@ -39,6 +39,63 @@ const RESUME_MIN_S = 30;
 const RESUME_MAX_FRACTION = 0.97;
 const QUEUE_HIGH_WATER = 12;
 
+// ffprobe reports a container's language tag as ISO 639-2, and in either of
+// its two variants for the dozen languages that have both — a bibliographic
+// one (fre, ger, dut) and a terminological one (fra, deu, nld), with real
+// files in this library using each. `Intl.DisplayNames` wants 639-1, so both
+// variants are folded onto the same two-letter code here. Only what a media
+// container actually carries is listed; anything unmapped falls through to the
+// raw tag, which is more useful than "Unknown".
+const _ISO639 = {
+  ara: 'ar', ben: 'bn', bul: 'bg', cat: 'ca', ces: 'cs', cze: 'cs',
+  chi: 'zh', dan: 'da', deu: 'de', dut: 'nl', ell: 'el', eng: 'en',
+  est: 'et', fas: 'fa', fin: 'fi', fra: 'fr', fre: 'fr', ger: 'de',
+  gle: 'ga', gre: 'el', heb: 'he', hin: 'hi', hrv: 'hr', hun: 'hu',
+  ice: 'is', ind: 'id', isl: 'is', ita: 'it', jpn: 'ja', kor: 'ko',
+  lav: 'lv', lit: 'lt', may: 'ms', msa: 'ms', nld: 'nl', nor: 'no',
+  per: 'fa', pol: 'pl', por: 'pt', ron: 'ro', rum: 'ro', rus: 'ru',
+  slk: 'sk', slo: 'sk', slv: 'sl', spa: 'es', srp: 'sr', swe: 'sv',
+  tam: 'ta', tha: 'th', tur: 'tr', ukr: 'uk', urd: 'ur', vie: 'vi',
+  zho: 'zh',
+};
+
+/**
+ * What to call one audio track, in the reader's language.
+ *
+ * The container's own `title` tag is preferred when there is one: a muxer that
+ * bothered to write "VFQ" or "Director's commentary" has said something the
+ * language code cannot, and two tracks tagged with the same language are
+ * otherwise indistinguishable in the menu — which is common, since a stereo
+ * downmix usually sits beside the surround track it came from.
+ */
+function audioTrackLabel(track) {
+  const code = (track.lang || '').toLowerCase();
+  let name = null;
+  const iso = _ISO639[code] || (code.length === 2 ? code : null);
+  if (iso) {
+    try {
+      name = new Intl.DisplayNames([getLocale()], { type: 'language' }).of(iso);
+      // `Intl.DisplayNames` follows each locale's prose convention, which is
+      // lower case in French, Spanish and Italian among others. A menu entry
+      // is not prose, and "français" beside "AC3 5.1" reads like a bug. Only
+      // this branch needs it: a raw tag is a code and is shown as written,
+      // and the numbered fallback comes from the catalogues already cased.
+      if (name) name = name.charAt(0).toUpperCase() + name.slice(1);
+    } catch { /* no Intl.DisplayNames, or a code it does not know */ }
+  }
+  if (!name && code && code !== 'und') name = code;
+  if (!name) name = t('video.audio_track_n', { n: track.i + 1 });
+  // Two tracks in the same language are one menu entry repeated without
+  // this, and a library where a stereo downmix sits beside the surround
+  // track it came from is the ordinary case. The container's title wins; the
+  // channel layout is the fallback, written in the "5.1" notation that needs
+  // no catalogue entry in any of the ten languages.
+  let detail = track.title;
+  if (!detail && track.ch > 2) detail = `${track.ch - 1}.1`;
+  else if (!detail && track.ch) detail = `${track.ch}.0`;
+  return detail ? `${name} — ${detail}` : name;
+}
+
 function _mseSupported(codec) {
   if (!window.MediaSource) return false;
   const mime = `video/mp4; codecs="${codec}"`;
@@ -151,6 +208,16 @@ function VideoPlayer({ entry, transportRef, gekRef, onClose, onDownload }) {
   // live; the render needs to reach it for "start from the beginning".
   const requestSeekRef = useRef(null);
   const [resumedFrom, setResumedFrom] = useState(0);
+  // The audio tracks this node reported for this file, and which one is
+  // playing. An empty list means either a file with one track or a node too
+  // old to enumerate them — both draw no selector, which is why nothing here
+  // needs to know which of the two it is.
+  const [audioTracks, setAudioTracks] = useState([]);
+  const [audioTrack, setAudioTrack] = useState(0);
+  const [audioMenuOpen, setAudioMenuOpen] = useState(false);
+  // Read inside the effect's closures, which are built once and would
+  // otherwise capture the first track forever.
+  const audioTrackRef = useRef(null);
   const [castActive, setCastActive] = useState(false);
   const [castUrl, setCastUrl] = useState(null);
   const [castPickerOpen, setCastPickerOpen] = useState(false);
@@ -412,7 +479,7 @@ function VideoPlayer({ entry, transportRef, gekRef, onClose, onDownload }) {
         outstandingRef.current = STREAM_WINDOW;
         setPhase('loading');
         console.log('[seek] request', +target.toFixed(1), 'outstanding:', STREAM_WINDOW);
-        t.requestStream(entry.id, STREAM_WINDOW, target);
+        t.requestStream(entry.id, STREAM_WINDOW, target, audioTrackRef.current);
       }, SEEK_DEBOUNCE_MS);
     };
     requestSeekRef.current = requestSeek;
@@ -557,6 +624,16 @@ function VideoPlayer({ entry, transportRef, gekRef, onClose, onDownload }) {
         const mime = `video/mp4; codecs="${msg.codec}"`;
         castCodecRef.current = msg.codec;
 
+        // What the node offered, and what it actually used — which is not
+        // always what was asked for: a file replaced on disk since the list
+        // was drawn falls back to the first track, and the selector must show
+        // the truth rather than the request.
+        setAudioTracks(Array.isArray(msg.audio_tracks) ? msg.audio_tracks : []);
+        if (Number.isInteger(msg.audio_track)) {
+          audioTrackRef.current = msg.audio_track;
+          setAudioTrack(msg.audio_track);
+        }
+
         if (!window.MediaSource || !MediaSource.isTypeSupported(mime)) {
           setError(t('video.err_mse', { codec: msg.codec }));
           setPhase('error');
@@ -769,7 +846,7 @@ function VideoPlayer({ entry, transportRef, gekRef, onClose, onDownload }) {
       outstandingRef.current = STREAM_WINDOW;
       const resumeAt = readResumePosition(entry.id);
       if (resumeAt) setResumedFrom(resumeAt);
-      transport.requestStream(entry.id, STREAM_WINDOW, resumeAt);
+      transport.requestStream(entry.id, STREAM_WINDOW, resumeAt, audioTrackRef.current);
     };
 
     // Closing the tab, or backgrounding it on a phone, never runs a React
@@ -954,6 +1031,40 @@ function VideoPlayer({ entry, transportRef, gekRef, onClose, onDownload }) {
     }}>
       <div class="video-top-bar">
         <span class="video-title">${entry.name} (${formatSize(entry.size)})</span>
+        ${audioTracks.length > 1 && html`
+          <div class="cast-wrapper" style="position:relative">
+            <button class="video-close ${audioMenuOpen ? 'cast-active' : ''}"
+              onClick=${() => setAudioMenuOpen(!audioMenuOpen)}
+              title="${t('video.audio_track')}">
+              <${Icon} name="volume" /></button>
+            ${audioMenuOpen && html`
+              <div class="cast-picker">
+                ${audioTracks.map((track) => html`
+                  <button class="cast-picker-item" onClick=${() => {
+                    setAudioMenuOpen(false);
+                    if (track.i === audioTrackRef.current) return;
+                    // A different track is a different ffmpeg, so this is the
+                    // seek path verbatim — and it has to be, because the new
+                    // stream opens with an init segment the SourceBuffer can
+                    // only accept after the abort/remove that `reinitAt` does.
+                    // Resuming where the film already was is the whole point:
+                    // the viewer changed language, not position.
+                    audioTrackRef.current = track.i;
+                    setAudioTrack(track.i);
+                    const v = videoRef.current;
+                    const seek = requestSeekRef.current;
+                    if (v && seek) seek(v.currentTime);
+                  }}>
+                    ${track.i === audioTrack
+                      ? html`<${Icon} name="check" />`
+                      : html`<span style="display:inline-block;width:14px"></span>`}
+                    ${' '}${audioTrackLabel(track)}
+                  </button>
+                `)}
+              </div>
+            `}
+          </div>
+        `}
         ${platform.capabilities.lanCast && html`
           <div class="cast-wrapper" style="position:relative">
             <button class="video-close ${castActive ? 'cast-active' : ''}"
diff --git a/packages/meshbay-hub/tests/test_video_audio_track.py b/packages/meshbay-hub/tests/test_video_audio_track.py
new file mode 100644
index 0000000..534da6d
--- /dev/null
+++ b/packages/meshbay-hub/tests/test_video_audio_track.py
@@ -0,0 +1,213 @@
+"""
+Choosing the audio track from the player.
+
+A dubbed film carries several audio tracks and the node used to map the first
+one unconditionally, so it played in whichever language was muxed first. Three
+things have to hold on this side:
+
+**The selector exists only where the node said there was something to choose.**
+It is drawn from the `audio_tracks` list in `stream_init` and from nothing else
+— there is no version check in the player — so a node too old to enumerate them
+draws no selector and is never sent an `audio_track` it would ignore. Ignoring
+it would not degrade the stream; it would serve a different language in
+silence, which is the failure this shape exists to make impossible.
+
+**Changing track is the seek path.** One ffmpeg produces one audio track, so
+there is nothing to switch inside a running stream: the node has to be asked
+again, and the new stream opens with an init segment the SourceBuffer can only
+accept after `reinitAt`'s abort/remove.
+
+**The label is derived, not translated.** ffprobe reports ISO 639-2 in either
+of its two variants, and the ten catalogues have no room for a language list —
+`Intl.DisplayNames` does that, from a fold that is real logic and is run here
+rather than read.
+"""
+
+import json
+import shutil
+import subprocess
+from pathlib import Path
+
+import pytest
+
+STATIC = Path(__file__).resolve().parents[1] / "src" / "meshbay_hub" / "static"
+APP = STATIC / "video-player.js"
+TRANSPORT = STATIC / "transport.js"
+
+pytestmark = pytest.mark.skipif(
+    shutil.which("node") is None or not APP.exists(),
+    reason="node or the SPA sources are not available")
+
+
+@pytest.fixture(scope="module")
+def app():
+    return APP.read_text()
+
+
+def _player(app: str) -> str:
+    i = app.index("function VideoPlayer(")
+    nxt = app.find("\nfunction ", i + 1)
+    return app[i:nxt if nxt > 0 else len(app)]
+
+
+def _lift(app: str, name: str) -> str:
+    """One top-level function, as text, for node to execute."""
+    start = app.index(f"function {name}(")
+    depth, i, seen = 0, start, False
+    while i < len(app):
+        if app[i] == "{":
+            depth += 1
+            seen = True
+        elif app[i] == "}":
+            depth -= 1
+            if seen and depth == 0:
+                return app[start:i + 1]
+        i += 1
+    raise AssertionError(f"{name} never closes")
+
+
+# ── The label, run rather than read ───────────────────────────────────────────
+
+def _label_cases(tmp_path, app, cases, locale="en"):
+    script = tmp_path / "label.mjs"
+    src = "\n".join([
+        app[app.index("const _ISO639 = {"):app.index("};", app.index("const _ISO639 = {")) + 2],
+        _lift(app, "audioTrackLabel"),
+    ])
+    script.write_text(
+        # The environment is modelled; the function under test is the shipped
+        # text above. `t` is only reached for a track with no usable language.
+        f"const getLocale = () => '{locale}';\n"
+        "const t = (k, p) => `${k}:${p.n}`;\n"
+        + src
+        + "\nconst out = JSON.parse(process.argv[2]).map(audioTrackLabel);\n"
+          "console.log(JSON.stringify(out));\n")
+    proc = subprocess.run(
+        ["node", str(script), json.dumps(cases)],
+        capture_output=True, text=True)
+    assert proc.returncode == 0, proc.stderr
+    return json.loads(proc.stdout)
+
+
+def test_both_iso_639_2_variants_name_the_same_language(tmp_path, app):
+    """Real files in one library use each: `fre` and `fra` are both French,
+    `ger` and `deu` both German. A fold that covers only one variant leaves
+    half a collection labelled with a three-letter code."""
+    out = _label_cases(tmp_path, app, [
+        {"i": 0, "lang": "fre", "ch": 2},
+        {"i": 1, "lang": "fra", "ch": 2},
+        {"i": 2, "lang": "ger", "ch": 2},
+        {"i": 3, "lang": "deu", "ch": 2},
+    ])
+    assert out[0] == out[1], f"fre and fra must agree: {out}"
+    assert out[2] == out[3], f"ger and deu must agree: {out}"
+    assert "French" in out[0] and "German" in out[2], out
+
+
+def test_the_container_title_wins_over_the_language_name(tmp_path, app):
+    """Two tracks in one language are the same menu entry twice without it."""
+    out = _label_cases(tmp_path, app, [
+        {"i": 0, "lang": "fre", "title": "Surround", "ch": 6},
+        {"i": 1, "lang": "fre", "title": "Stereo", "ch": 2},
+    ])
+    assert out[0] != out[1]
+    assert "Surround" in out[0] and "Stereo" in out[1]
+
+
+def test_an_untitled_track_falls_back_to_its_channel_layout(tmp_path, app):
+    """Still two distinguishable entries, and "5.1" needs no catalogue entry."""
+    out = _label_cases(tmp_path, app, [
+        {"i": 0, "lang": "fre", "ch": 6},
+        {"i": 1, "lang": "fre", "ch": 2},
+    ])
+    assert out[0] != out[1], f"two French tracks must not read alike: {out}"
+    assert "5.1" in out[0] and "2.0" in out[1], out
+
+
+def test_an_untagged_track_is_numbered_not_called_unknown(tmp_path, app):
+    """`und`, or no tag at all — common enough in real files, and a number the
+    viewer can act on beats a word that tells them nothing."""
+    out = _label_cases(tmp_path, app, [
+        {"i": 0, "lang": "und", "ch": 2},
+        {"i": 1, "ch": 2},
+    ])
+    assert out[0].startswith("video.audio_track_n:1"), out
+    assert out[1].startswith("video.audio_track_n:2"), out
+
+
+def test_a_language_the_fold_does_not_know_shows_its_tag(tmp_path, app):
+    """More useful than "Unknown": the tag is what the file actually says."""
+    out = _label_cases(tmp_path, app, [{"i": 0, "lang": "qaa", "ch": 2}])
+    assert "qaa" in out[0], out
+
+
+def test_a_language_name_is_cased_for_a_menu_not_for_prose(tmp_path, app):
+    """`Intl.DisplayNames` returns "français", which is right in a sentence and
+    wrong in a menu beside "AC3 5.1". The locales where this bites are the ones
+    this project ships most of."""
+    out = _label_cases(tmp_path, app, [
+        {"i": 0, "lang": "fre", "ch": 6},
+        {"i": 1, "lang": "eng", "ch": 6},
+    ], locale="fr")
+    assert out == ["Français — 5.1", "Anglais — 5.1"], out
+
+
+# ── The capability gate ───────────────────────────────────────────────────────
+
+def test_the_selector_is_drawn_from_the_nodes_list_and_no_version(app):
+    """The whole negotiation. A version comparison anywhere near this is the
+    bug: the floor is below the current version, so an older node is reachable,
+    and the only honest signal that it can switch track is that it enumerated."""
+    player = _player(app)
+    assert "audioTracks.length > 1" in player, \
+        "the selector must be gated on the list the node sent"
+    menu = player[player.index("audioTracks.length > 1"):]
+    menu = menu[:menu.index("platform.capabilities.lanCast")]
+    for forbidden in ("MNP_VERSION", "peerVersion", "3.2", "v_min"):
+        assert forbidden not in menu, (
+            f"the selector branches on {forbidden}, not on what the node said")
+
+
+def test_no_track_is_ever_sent_to_a_node_that_did_not_offer_one():
+    """`requestStream` must omit the field rather than default it.
+
+    A node that ignores `audio_track` does not degrade — it serves a different
+    language and says nothing. Sending 0 "harmlessly" to every node is what
+    makes that reachable, so the field has to be absent unless the caller was
+    given a real one.
+    """
+    src = TRANSPORT.read_text()
+    fn = src[src.index("requestStream(fileId"):]
+    fn = fn[:fn.index("\n  /** Room for")]
+    assert "Number.isInteger(audioTrack)" in fn, \
+        "an omitted track must not become 0 on the wire"
+    assert "req.audio_track = audioTrack" in fn
+    assert "audio_track: audioTrack" not in fn, \
+        "the field must be added conditionally, never built into the literal"
+
+
+def test_changing_track_restarts_the_stream_where_the_film_already_was(app):
+    """Setting the ref alone changes nothing: one ffmpeg carries one audio
+    track, so the node has to be asked again — and at the current position,
+    because the viewer changed language and not place."""
+    player = _player(app)
+    handler = player[player.index("audioTracks.length > 1"):]
+    handler = handler[:handler.index("platform.capabilities.lanCast")]
+    assert "audioTrackRef.current = track.i" in handler
+    assert "requestSeekRef.current" in handler, \
+        "a track change must go through the seek path"
+    assert "seek(v.currentTime)" in handler, \
+        "a track change must resume where the film already was"
+
+
+def test_the_player_believes_the_node_about_which_track_is_playing(app):
+    """`stream_init` reports the track actually used, which is not always the
+    one asked for — a list drawn before the file was replaced on disk can name
+    a track that is gone, and the node falls back to the first. Showing the
+    request instead of the answer would tick the wrong entry for the rest of
+    the film."""
+    player = _player(app)
+    assert "Number.isInteger(msg.audio_track)" in player
+    init = player[player.index("Number.isInteger(msg.audio_track)"):]
+    init = init[:init.index("}")]
+    assert "audioTrackRef.current = msg.audio_track" in init