[fix](trx-frontend-http): route vchan commands to correct rig in background decode

BackgroundDecodeManager.send_audio_cmd used the global active_rig_id()
to route virtual channel commands. During a rig switch, Remove commands
for the old rig's channels were sent to the new rig's audio pipeline,
leaving orphaned virtual channels on the previous rig's server.

Replace send_audio_cmd with send_audio_cmd_to_rig that takes an explicit
rig_id, derived from the channel's own rig_id field. Both Remove and
SubscribeBackground commands now reach the correct rig.

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

src/trx-client/trx-frontend/trx-frontend-http/src/background_decode.rs

@@ -254,16 +254,14 @@ impl BackgroundDecodeManager {
             .and_then(|guard| guard.clone())
     }
 
-    fn send_audio_cmd(&self, cmd: VChanAudioCmd) {
+    fn send_audio_cmd_to_rig(&self, rig_id: &str, cmd: VChanAudioCmd) {
         // Route through per-rig sender when available.
-        if let Some(rig_id) = self.active_rig_id() {
         if let Ok(map) = self.context.vchan.rig_audio_cmd.read() {
-                if let Some(tx) = map.get(&rig_id) {
+            if let Some(tx) = map.get(rig_id) {
                 let _ = tx.try_send(cmd);
                 return;
             }
         }
-        }
         // Fall back to global sender.
         if let Ok(guard) = self.context.vchan.audio_cmd.lock() {
             if let Some(tx) = guard.as_ref() {
@@ -273,7 +271,7 @@ impl BackgroundDecodeManager {
     }
 
     fn remove_channel(&self, channel: &VirtualBackgroundDecodeChannel) {
-        self.send_audio_cmd(VChanAudioCmd::Remove(channel.uuid));
+        self.send_audio_cmd_to_rig(&channel.rig_id, VChanAudioCmd::Remove(channel.uuid));
     }
 
     fn clear_runtime_channels(&self, runtime: &mut BackgroundRuntimeState) {
@@ -444,13 +442,16 @@ impl BackgroundDecodeManager {
             if runtime.active_channels.contains_key(&bookmark_id) {
                 continue;
             }
-            self.send_audio_cmd(VChanAudioCmd::SubscribeBackground {
+            self.send_audio_cmd_to_rig(
+                &desired.rig_id,
+                VChanAudioCmd::SubscribeBackground {
                     uuid: desired.uuid,
                     freq_hz: desired.freq_hz,
                     mode: desired.mode.clone(),
                     bandwidth_hz: desired.bandwidth_hz,
                     decoder_kinds: desired.decoder_kinds.clone(),
-            });
+                },
+            );
             runtime.active_channels.insert(bookmark_id, desired);
         }