[fix](trx-client,trx-server): fix spectrum death and stuck shutdown after IQ overflow

Two bugs triggered by a SoapySDR IQ overflow:

1. Spectrum dies permanently (trx-client): when GetSpectrum times out
   (300ms SPECTRUM_IO_TIMEOUT), the error was silently swallowed and
   the spectrum buffer cleared. The in-flight response remained in the
   TCP receive buffer, desynchronising all subsequent reads so every
   poll kept failing. Fix: propagate the error so handle_connection
   returns and the outer loop reconnects, restoring TCP sync.

2. CTRL+C hangs trx-server: after IQ overflow, the sdr-iq-read thread
   can get stuck in a blocking SoapySDR/USB call (deactivate/activate
   with no timeout). Tokio received SIGINT and aborted async tasks, but
   the process could not exit while the native thread was blocked in
   uninterruptible I/O. Fix: call std::process::exit(0) after the
   graceful shutdown sequence so the OS forcibly terminates all threads.

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

src/trx-client/src/remote_client.rs

@@ -183,11 +183,15 @@ async fn handle_connection(
                             guard.replace(snapshot.spectrum);
                         }
                     }
-                    Err(_) => {
-                        // Backend may not support spectrum; clear buffer silently.
+                    Err(e) => {
+                        // A spectrum poll failure desynchronises the TCP stream
+                        // (the in-flight response is still in the buffer).
+                        // Propagate the error so the caller reconnects and
+                        // restores protocol sync.
                         if let Ok(mut guard) = config.spectrum.lock() {
                             guard.replace(None);
                         }
+                        return Err(e);
                     }
                 }
             }