[feat](trx-rs): per-virtual-channel audio streaming

Add end-to-end audio routing for virtual DSP channels:

Server (trx-server):
- New wire protocol: AUDIO_MSG_RX_FRAME_CH (0x0b), VCHAN_ALLOCATED (0x0c),
  VCHAN_SUB (0x0d), VCHAN_UNSUB (0x0e), VCHAN_FREQ (0x0f), VCHAN_MODE (0x10),
  VCHAN_REMOVE (0x11) frame types in trx-core audio.rs
- Add frame helpers: write_vchan_uuid_msg, write_vchan_audio_frame,
  parse_vchan_audio_frame, parse_vchan_uuid_msg
- Add ensure_channel_pcm() to VirtualChannelManager trait; implement in
  SdrVirtualChannelManager with create-or-subscribe semantics using client UUID
- Extend audio.rs handle_audio_client: VChanCmd dispatcher, per-channel Opus
  encoder tasks, VCHAN_SUB/UNSUB/FREQ/MODE/REMOVE reader loop handlers
- Thread vchan_manager through run_audio_listener / spawn_rig_audio_stack

Client (trx-client):
- Add VChanAudioCmd enum to trx-frontend; add vchan_audio and vchan_audio_cmd
  fields to FrontendRuntimeContext
- Extend audio_client: demux AUDIO_MSG_RX_FRAME_CH to per-channel broadcasters,
  handle VCHAN_ALLOCATED; forward VChanAudioCmd over TCP write loop
- Wire vchan_cmd_tx/rx channel in main.rs; pass vchan_audio map to audio_client
- ClientChannelManager.set_audio_cmd() / send_audio_cmd(): dispatch
  Subscribe/Remove/SetFreq/SetMode on allocate/delete/freq/mode operations
- Wire audio_cmd sender in server.rs serve() after creating vchan_mgr

HTTP frontend:
- /audio?channel_id=<uuid>: route WebSocket to per-channel Opus broadcaster
- vchan.js: vchanReconnectAudio() stops/restarts RX audio on channel switch;
  _audioChannelOverride in app.js selects primary vs virtual WS endpoint
- app.js: _audioChannelOverride variable; startRxAudio appends channel param

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

Cargo.lock

@@ -2535,6 +2535,7 @@ dependencies = [
  "trx-frontend-http-json",
  "trx-frontend-rigctl",
  "trx-protocol",
+ "uuid",
 ]
 
 [[package]]
@@ -2576,6 +2577,7 @@ dependencies = [
  "serde_json",
  "tokio",
  "trx-core",
+ "uuid",
 ]
 
 [[package]]
@@ -2678,6 +2680,7 @@ dependencies = [
  "trx-protocol",
  "trx-vdes",
  "trx-wspr",
+ "uuid",
 ]
 
 [[package]]