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, 26 insertions, 7 deletions

diff --git a/docs/MESHBAY_DESIGN.md b/docs/MESHBAY_DESIGN.md
index a079e50..e7d4a76 100644
--- a/docs/MESHBAY_DESIGN.md
+++ b/docs/MESHBAY_DESIGN.md
@@ -1102,19 +1102,30 @@ client on reload; the desktop client ships its own UI, which is why
 connecting and says "this version can no longer connect" rather than showing a
 handshake refusal nobody can act on.
 
-**The floor being the current version is what keeps capability branches out of the
-client.** `MNP_MIN_SUPPORTED` equals `MNP_VERSION`, so `check_version` refuses
-every older peer at the handshake — which means **every capability is true of every
-peer the client can reach**, and there is nothing to test for. An upload is sealed
-or it is not sent; a transfer has a real lease or it does not run; there is one
-app-directories op and no wrappers behind it. The client records the version its
-peer declared, for diagnostics, and **branches on none of it**.
+**Every *requirement* is true of every peer the client can reach.** The floor moves
+with each MAJOR, so `check_version` refuses at the handshake any peer that cannot
+meet one: an upload is sealed or it is not sent; a transfer has a real lease or it
+does not run; there is one app-directories op and no wrappers behind it. The client
+records the version its peer declared, for diagnostics, and **branches on none of
+it**.
 
 > A capability flag on a peer whose floor already guarantees the capability is a
 > branch that can only ever take one path — until somebody lowers the floor, at
 > which point it silently takes the other. **A field kept "just in case" is how
 > the branches come back.**
 
+**The floor is not the current version, and MINOR additions are why.** It is
+`MNP_MIN_SUPPORTED` in `handshake.py`, it equals the last MAJOR, and 3.1 and 3.2
+have both been added above it without moving it. So a peer can be reachable and
+still not do something the current version can, and the client has to cope with
+that — **by reading the peer's own answer, never by comparing version numbers**.
+3.2's audio tracks are the worked example: the node lists them in `stream_init`,
+the client draws its selector from that list, and a node that sends no list gets no
+selector. That is not the branch the box above refuses. The branch it refuses is a
+flag the client sets from a version it parsed; this is the node stating what it
+has, in the same message the feature already needed, and it takes exactly one path
+per peer because the peer said which.
+
 Where a break leaves data behind, a migration runs with the node stopped, backs
 the database up first and is idempotent. But **a migration that has to be
 remembered is a migration that does not happen**, so anything that *can* be a
@@ -2020,6 +2031,14 @@ MSE string) fed to a source buffer, with the node holding one slot per viewer.
 - **Seeking restarts the source with an index seek before the input**, clamped away
   from the end and echoed back; the client supplies the timestamp offset, because
   copying timestamps does not preserve position.
+- **The viewer picks the audio track, and picking one is a seek.** One ffmpeg
+  carries one audio track, so there is nothing to switch inside a running stream:
+  the node is asked again at the current position and the source buffer is reset
+  the way any seek resets it. It costs nothing extra — audio is transcoded on every
+  stream anyway — and mapping the first track unconditionally, which is what this
+  replaced, made a dubbed library playable in one language only. The node reports
+  the tracks it found and the one it used; **the client draws its selector from
+  that list and from no version number**, which is what keeps the addition MINOR.
 - **Losing a peer must stop its work, not merely forget it** — anything holding a
   resource is shut down on the way out, or a closed tab transcodes for the length of
   the credit timeout.