feat: implement bind feature for associating channels with MIDI notes

Co-authored-by: aider (deepseek/deepseek-reasoner) <aider@aider.chat>

src/looper.c

@@ -18,6 +18,7 @@ atomic_int  cmd_remove = 0;
 jack_port_t *midi_control_port = NULL;
 jack_port_t *midi_clock_port = NULL;
 atomic_int  control_key_active = 0;
+atomic_int  bind_channel = 0;
 
 /* Deferred removal index (1 second grace) */
 static int pending_unregister_idx = -1;