Add PixelStomp MUX driver (DAT=9, LD=10, CLK=11, DI=12)

- New PixelStompMux class with shift register read/write
- Read 10 buttons via DI (active LOW)
- Drive 10 LEDs via DAT (shift out + latch)
- LED startup animation uses MUX
- Switch polling reads from MUX shift register
- MUX instance shared between switch and LED drivers

src/main.cpp

@@ -3,10 +3,13 @@
 
 #include <Arduino.h>
 #include "midi_transport.h"
+#include "pixel_stomp_mux.h"
 #include "led_stub.h"
 #include "switch_stub.h"
 #include "app_task.h"
 
+PixelStompMux mux(9, 10, 11, 12);
+
 DefaultLedStub led_driver;
 DefaultSwitchStub switch_driver;
 UsbMidiTransport midi_transport;
@@ -34,10 +37,15 @@ void setup() {
     Serial.println("  Board: ESP32-S3-WROOM-1");
     Serial.println("=================================");
     
+    Serial.println("[INIT] Initializing PixelStomp MUX...");
+    mux.begin();
+    
     Serial.println("[INIT] Starting LED startup animation...");
+    led_driver.set_mux(&mux);
     led_driver.begin();
     
-    Serial.println("[INIT] Initializing switches...");
+    Serial.println("[INIT] Initializing switches via MUX...");
+    switch_driver.set_mux(&mux);
     switch_driver.begin();
     
     Serial.println("[INIT] Initializing USB MIDI...");