[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>

src/trx-client/trx-frontend/trx-frontend-http/assets/web/app.js

@@ -5879,6 +5879,9 @@ function extractAudioFrameChannels(frame) {
   return out;
 }
 
+// Optional channel_id injected by vchan.js when connecting to a virtual channel.
+let _audioChannelOverride = null;
+
 function startRxAudio() {
   if (rxActive) { stopRxAudio(); return; }
   if (!hasWebCodecs) {
@@ -5886,7 +5889,10 @@ function startRxAudio() {
     return;
   }
   const proto = location.protocol === "https:" ? "wss:" : "ws:";
-  audioWs = new WebSocket(`${proto}//${location.host}/audio`);
+  const audioPath = _audioChannelOverride
+    ? `/audio?channel_id=${encodeURIComponent(_audioChannelOverride)}`
+    : "/audio";
+  audioWs = new WebSocket(`${proto}//${location.host}${audioPath}`);
   audioWs.binaryType = "arraybuffer";
   audioStatus.textContent = "Connecting…";
 

src/trx-client/trx-frontend/trx-frontend-http/assets/web/plugins/vchan.js

@@ -36,10 +36,11 @@ function vchanHandleChannels(data) {
     const d = JSON.parse(data);
     vchanRigId = d.rig_id || null;
     vchanChannels = d.channels || [];
-    // If the active channel was evicted, fall back to channel 0.
+    // If the active channel was evicted, fall back to channel 0 and reconnect audio.
     const ids = new Set(vchanChannels.map(c => c.id));
     if (vchanActiveId && !ids.has(vchanActiveId)) {
       vchanActiveId = vchanChannels.length > 0 ? vchanChannels[0].id : null;
+      vchanReconnectAudio();
     }
     vchanRender();
   } catch (e) {
@@ -120,6 +121,7 @@ async function vchanAllocate() {
     // The SSE `channels` event will trigger vchanRender(); optimistically
     // mark active so the picker feels responsive even before the event arrives.
     vchanRender();
+    vchanReconnectAudio();
   } catch (e) {
     console.error("vchan: allocate error", e);
   }
@@ -159,11 +161,30 @@ async function vchanSubscribe(channelId) {
     vchanActiveId = channelId;
     vchanRender();
     vchanSyncModeDisplay();
+    vchanReconnectAudio();
   } catch (e) {
     console.error("vchan: subscribe error", e);
   }
 }
 
+// Reconnect the audio WebSocket to the appropriate endpoint:
+// - virtual channel: /audio?channel_id=<uuid>
+// - primary channel: /audio (no param)
+// Only reconnects if RX audio is currently active.
+function vchanReconnectAudio() {
+  if (typeof rxActive === "undefined" || !rxActive) return;
+  // Set the channel override so startRxAudio picks up the right URL.
+  const ch = vchanIsOnVirtual() ? vchanActiveChannel() : null;
+  if (typeof _audioChannelOverride !== "undefined") {
+    _audioChannelOverride = ch ? ch.id : null;
+  }
+  if (typeof stopRxAudio === "function") stopRxAudio();
+  // Small delay so the server has time to set up the per-channel encoder.
+  setTimeout(() => {
+    if (typeof startRxAudio === "function") startRxAudio();
+  }, 200);
+}
+
 // Called by app.js from applyCapabilities().
 // Shows the channel picker only for SDR rigs.
 function vchanApplyCapabilities(caps) {