feat: embedded subtitles in the video player (MNP 3.3)

MSE decodes no in-band text track, so a subtitle cannot ride inside the
fragmented MP4 the player is fed. The node extracts one track whole, converts
it to WebVTT and caches it under its own hash; the client pulls that blob
through the ordinary file_req/chunk path and hangs a <track> on the video
element — the same indirection as a TMDB poster or an audio transcode, which
is what makes a film's subtitles extracted once in the life of the file rather
than once per viewing. Whole-file also makes the cues absolute, so a seek and
an audio-language change both leave the track untouched.

**The ordinal counts every subtitle stream, including the ones never listed.**
Only text codecs are offered: a bitmap track (PGS, VOBSUB — about a fifth of a
real library) has no path to WebVTT without OCR, and one extracted anyway
yields a header with no cues, which is a menu entry that shows nothing and
reports no error. Numbering the survivors of that filter would give a
PGS/SRT/SRT file the ordinals 0 and 1 for its text tracks and `-map 0:s:0`
would then extract the PGS — the same trap `AudioTrack.ordinal` exists for,
one level deeper. A fixture whose first subtitle stream cannot be decoded pins
it, and the handler checks membership of the probed list, never a range.

Additive and MINOR: the selector is drawn from `subtitle_tracks` in the node's
own `stream_init` and from no version number, so `subtitle_req` is never sent
to a peer that would not answer it. The floor stays at 3.0.

Also here: a failed extraction never touches playback, a superseded reply
cannot install its blob over a newer choice, and `_languageName` is shared
with the audio labels — lifted by both label harnesses, since a lift that
names one function stops covering the rule the moment logic moves out of it.

Tests: 9 node (tracks told apart by the words in the extracted cues, not by
tags), 10 client. Full suite green: 1545 node/common, 1252 hub.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UGY17EPph5LsLzePPXhUVc

1 files changed, 37 insertions, 0 deletions

diff --git a/packages/meshbay-hub/src/meshbay_hub/static/transport.js b/packages/meshbay-hub/src/meshbay_hub/static/transport.js
index 262b00e..63c71ff 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/transport.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/transport.js
@@ -1607,6 +1607,25 @@ class MeshBayTransport {
   }
 
   /**
+   * One embedded subtitle track, extracted node-side to WebVTT. Returns
+   * `{ hash, size, mime, track }` — the *cache* hash to pull through the
+   * ordinary file_req/chunk path, the same indirection as an audio transcode
+   * or a TMDB poster, and cached node-side under the file's own id so a film
+   * is extracted once rather than once per viewing.
+   *
+   * `track` is the ordinal the node published in `stream_init.subtitle_tracks`
+   * and is passed back untouched: it counts every subtitle stream in the
+   * container, including the bitmap ones that are never listed, so it is not
+   * a position in the list this client received.
+   */
+  async requestSubtitle(fileId, track) {
+    const msg = await this._sendAndWait(
+      { type: 'subtitle_req', v: '0.9', file_id: fileId, track }, 90000);
+    if (msg.type === 'error') throw new Error(msg.detail);
+    return msg;
+  }
+
+  /**
    * Whether MusicBrainz lookups run for this group at all — per-group from
    * the start (docs/musicbay.md §3.2/§6). Signed like setTmdbEnabled.
    */
@@ -2968,6 +2987,12 @@ class MeshBayTransport {
           // Same reordering hazard as media_meta_req: the player prefetches
           // the next track while the current one may still be transcoding.
           : obj.type === 'audio_transcode_req' ? `audio_transcode:${obj.file_id}`
+          // The track ordinal is part of the key, not just the file id: a
+          // viewer who opens the menu and picks a second language before the
+          // first extraction has answered has two of these in flight for the
+          // same film, and the one that arrives first is not necessarily the
+          // one that was asked for first.
+          : obj.type === 'subtitle_req' ? `subtitle:${obj.file_id}:${obj.track}`
           // Two-step admin-op flow (_authorizeAdminOp) — see ADMIN_OP_TYPES'
           // own comment for the race this closes. The initial request and
           // the admin_response that follows it are keyed the same way
@@ -3399,6 +3424,18 @@ class MeshBayTransport {
       return;
     }
 
+    // Keyed on file *and* track — see the `subtitle_req` key above. The node
+    // echoes `track` back for exactly this: without it a reply could only be
+    // matched to the film, and the two tracks of one film are precisely the
+    // pair that can be in flight together.
+    if (msg.type === 'subtitle_resp') {
+      const key = `subtitle:${msg.file_id}:${msg.track}`;
+      for (const [, handler] of this._pending) {
+        if (handler._key === key) { handler.resolve(msg); return; }
+      }
+      return;
+    }
+
     // Same reasoning as media_meta_resp: keyed, not arrival-order, and
     // "nobody's waiting any more" must not fall through either.
     if (msg.type === 'season_meta_resp') {