[feat](trx-frontend-rigctl): per-rig rigctl listeners for multi-rig setups

Add rig_ports map to RigctlFrontendConfig. When non-empty, one rigctl
TCP listener is spawned per entry instead of the single shared listener,
each routing commands to its assigned rig via rig_id_override on RigRequest.

Add rig_id_override: Option<String> to RigRequest so the remote client
can route individual requests to a specific rig without changing the
globally selected rig. build_envelope prefers the override when set.

Example config:
  [frontends.rigctl]
  enabled = true
  listen = "127.0.0.1"
  port = 4532
  rig_ports.ft817 = 4532
  rig_ports.airspyhf = 4533

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

src/trx-server/src/listener.rs

@@ -300,6 +300,7 @@ async fn handle_client(
         let req = RigRequest {
             cmd: rig_cmd,
             respond_to: resp_tx,
+            rig_id_override: None,
         };
 
         match time::timeout(IO_TIMEOUT, handle.rig_tx.send(req)).await {

src/trx-server/src/rig_task.rs

@@ -316,7 +316,7 @@ pub async fn run_rig_task(
                 }
 
                 // Process each request
-                while let Some(RigRequest { cmd, respond_to }) = batch.pop() {
+                while let Some(RigRequest { cmd, respond_to, .. }) = batch.pop() {
                     let cmd_label = format!("{:?}", cmd);
                     let log_command = !matches!(&cmd, RigCommand::GetSpectrum);
                     let started = Instant::now();