fix: _process_4cm handles 1D mono input gracefully

When routing mode is '4cm' but JACK provides mono audio (1D array),
the _process_4cm method crashes with IndexError on audio_in[0,:].
Treat mono input as guitar channel with silent return channel.

This happens when JACK is configured with 1 capture port (mono mode)
but routing_mode is set to '4cm' — e.g. after a settings sweep that
toggles routing_mode without changing JACK port config.

src/dsp/pipeline.py

@@ -718,6 +718,12 @@ class AudioPipeline:
             ch0 = Send output (to amp input)
             ch1 = Return output (to amp FX return)
         """
+        if audio_in.ndim == 1:
+            # Mono input in 4CM mode — treat entire input as guitar
+            # channel, create silent return channel
+            ch0 = audio_in.copy()
+            ch1 = np.zeros_like(audio_in)
+        else:
             ch0 = audio_in[0, :].copy()
             ch1 = audio_in[1, :].copy()