Rewrite MUX: WS2812C LEDs (NeoPixel) + 74HC165 buttons

PixelStomp MUX uses:
- WS2812C LEDs: one data line via DAT (GPIO 9), NeoPixel protocol
- 74HC165 shift register: LD/CLK/DI for reading 8 button states

Changes:
- Use Adafruit NeoPixel library for LED control
- Proper 74HC165 parallel-load shift-in for buttons
- 8 switches + 8 LEDs (was incorrectly 10)
- Diagnostic commands: dump, ledtest, red, green, blue, read

src/switch_stub.cpp

@@ -16,7 +16,7 @@ DefaultSwitchStub::DefaultSwitchStub() : initialized(false) {
 }
 
 void DefaultSwitchStub::begin() {
-    Serial.println("[SW] Using PixelStomp MUX for button input");
+    Serial.println("[SW] Using 74HC165 via PixelStomp MUX");
     initialized = true;
 }