Serial commands:
- dump: show button and LED state
- ledon/ledoff: toggle all LEDs
- ledtest: sequential LED test
- read: raw button read
- probe: check pin states and test LD toggle
- 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
ash