[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-server/src/main.rs

@@ -1017,5 +1017,7 @@ async fn main() -> DynResult<()> {
     for handle in task_handles {
         let _ = handle.await;
     }
-    Ok(())
+    // Force exit so that native threads stuck in blocking hardware I/O
+    // (e.g. SoapySDR/USB transfers in D-state) cannot prevent shutdown.
+    std::process::exit(0);
 }