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Also here: a failed extraction never touches playback, a superseded reply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, 6 insertions, 0 deletions

diff --git a/packages/meshbay-hub/tests/test_video_audio_track.py b/packages/meshbay-hub/tests/test_video_audio_track.py
index 534da6d..6eb98ba 100644
--- a/packages/meshbay-hub/tests/test_video_audio_track.py
+++ b/packages/meshbay-hub/tests/test_video_audio_track.py
@@ -72,6 +72,12 @@ 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],
+        # The language name is shared with `subtitleTrackLabel`, so it lives in
+        # a function of its own and has to be lifted alongside its caller. A
+        # lift that names one function stops exercising anything the moment
+        # logic moves out of it — here it throws, which is the good case; the
+        # bad one is a lift that still runs and no longer covers the rule.
+        _lift(app, "_languageName"),
         _lift(app, "audioTrackLabel"),
     ])
     script.write_text(