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/midi_transport.cpp

@@ -94,7 +94,6 @@ void UsbMidiTransport::send_cc(uint8_t channel, uint8_t cc, uint8_t value) {
     if (!initialized) return;
     uint8_t packet[4] = {0x0B, (uint8_t)(0xB0 | (channel - 1)), cc, value};
     usb_midi.writePacket(packet);
-    Serial.printf("[MIDI OUT] Ch:%d CC:%d:%d\n", channel, cc, value);
 }
 
 bool UsbMidiTransport::is_connected() {