[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-client/trx-frontend/trx-frontend-http-json/src/server.rs

@@ -156,6 +156,7 @@ async fn handle_client(
         let req = RigRequest {
             cmd: rig_cmd,
             respond_to: resp_tx,
+            rig_id_override: None,
         };
 
         match time::timeout(IO_TIMEOUT, tx.send(req)).await {