Fix button-to-MIDI latency: remove blocking serial, reduce debounce, faster loop

Latency sources fixed:
- debounce 50ms -> 5ms (switch_stub.cpp:15)
- Removed [MIDI OUT] Serial.printf from send_cc() (midi_transport.cpp:93-98)
- Removed [APP] Switch pressed/released prints from update() (app_task.cpp:39,43)
- Removed [APP] Switch -> CC print from process_switch_event() (app_task.cpp:140-143)
- loop() delay 10ms -> 1ms (main.cpp:390)

Serial output at 115200 baud was taking 2-5ms per printf call
in the critical button-to-MIDI path, adding 7-15ms+ total latency
per button press. All removed from hot path.

src/main.cpp

@@ -387,5 +387,5 @@ void loop() {
         handle_serial_command(cmd);
     }
 
-    delay(10);
+    delay(1);
 }