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, 41 insertions, 5 deletions

diff --git a/packages/meshbay-node/src/meshbay_node/transport/webrtc_server.py b/packages/meshbay-node/src/meshbay_node/transport/webrtc_server.py
index 99ba3c9..b39941d 100644
--- a/packages/meshbay-node/src/meshbay_node/transport/webrtc_server.py
+++ b/packages/meshbay-node/src/meshbay_node/transport/webrtc_server.py
@@ -5948,11 +5948,14 @@ class WebRTCPeerSession:
         file_hash = bytes.fromhex(entry.id)
 
         try:
-            codec_str, duration, has_audio, _width, _height, raw_video_codec = \
-                await _probe_video(str(file_path))
+            probe = await _probe_video(str(file_path))
         except Exception as e:
             self._send({"type": "error", "detail": f"Probe failed: {e}"})
             return
+        codec_str = probe.codec
+        duration = probe.duration
+        has_audio = probe.has_audio
+        raw_video_codec = probe.raw_codec_name
 
         # No video stream at all is the only thing this path cannot serve, and
         # it is the only thing refused here. A source with no MSE codec string
@@ -6051,8 +6054,23 @@ class WebRTCPeerSession:
             codec_str = "avc1.640029,mp4a.40.2" if has_audio else "avc1.640029"
         else:
             codec_args = ["-c:v", "copy"]
+        # Which audio track. A dubbed film carries several and the first one is
+        # not a neutral default — it is whatever the person who muxed the file
+        # happened to put first, which across a real library is overwhelmingly
+        # one language. Out of range falls back to the first rather than
+        # refusing: the client's list comes from a `stream_init` that may
+        # predate the file being replaced on disk, and a viewer who asked for
+        # the second track of a file that now has one wants the film, not an
+        # error. `stream_init` says which track was actually used, the same way
+        # it says which `start` was actually used and for the same reason.
+        try:
+            audio_track = int(msg.get("audio_track", 0) or 0)
+        except (TypeError, ValueError):
+            audio_track = 0
+        if not 0 <= audio_track < len(probe.audio_tracks):
+            audio_track = 0
         if has_audio:
-            map_args += ["-map", "0:a:0"]
+            map_args += ["-map", f"0:a:{audio_track}"]
             # Downmixed to stereo: a WEB-DL's 5.1 track becomes 6-channel AAC
             # with no "-ac", which ffprobe and VLC accept fine but which some
             # browsers' MSE decoder rejects outright once real fragments are
@@ -6081,6 +6099,22 @@ class WebRTCPeerSession:
             # this is what the client adds back (`SourceBuffer.timestampOffset`)
             # to put the fragments where they belong on the timeline.
             "start": start,
+            # The track list is how a client discovers that this node can
+            # switch language at all — there is no version check anywhere in
+            # the player. A node that does not send it gets no selector, and
+            # the client then never sends `audio_track` to a peer that would
+            # ignore it and serve the wrong language without saying so.
+            "audio_tracks": [
+                {
+                    "i": tr.ordinal,
+                    "lang": tr.language,
+                    "title": tr.title,
+                    "codec": tr.codec_name,
+                    "ch": tr.channels,
+                }
+                for tr in probe.audio_tracks
+            ],
+            "audio_track": audio_track if has_audio else None,
         })
 
         # A client that says nothing gets the old behaviour, which is why this
@@ -6097,8 +6131,10 @@ class WebRTCPeerSession:
         self._stream_started_at = time.monotonic()
         self._stream_segments = 0
         reason = "eof"
-        log.info("stream: stream_init sent file=%s paced=%s credits=%d start=%.1fs",
-                 file_id[:12], paced, self._stream_credit, start)
+        log.info("stream: stream_init sent file=%s paced=%s credits=%d start=%.1fs "
+                 "audio=%s/%d",
+                 file_id[:12], paced, self._stream_credit, start,
+                 audio_track if has_audio else "-", len(probe.audio_tracks))
         try:
             while True:
                 if paced and not await self._await_stream_credit():