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>

1 files changed, 15 insertions, 3 deletions

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. */