New buzzer.h/buzzer.c: background pthread drives gpio64 with three
patterns (GRASS 500/500ms, ALERT 300/200ms, COLLISION 100/100ms).
Integrated into fire_collision_warning(), fire_alert(), and grass
state machine in event_recorder.c; buzzer_init() called after
can_bus_init() in kp_firmware.c.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
miketsai
286291fcfd