Add hw_button driver (PC2 active-low, 20ms debounce) with gesture detection:
- Single short press + 500ms quiet -> BTN_EVENT_PARK
- SHORT+SHORT+LONG combo (within 3s) -> BTN_EVENT_REARM_COMBO
New BALANCE_PARKED state: PID frozen, motors off, quick re-arm via button
combo without the 3-second arm interlock required from DISARMED.
FC_BTN (0x404) CAN frame sent to Orin on each event:
event_id 1=PARKED, 2=UNPARKED, 3=UNPARK_FAILED (pitch > 20 deg)
Includes 11 unit tests (1016 assertions) exercising debounce, bounce
rejection, short/long classification, sequence detection, and timeout.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
