Two targeted fixes:
1. Deduplicate LED writes in handle_knob: compute new colours and only
call send_leds() when they differ from last_led_colors. A single
physical knob turn produces 20-50 ADC samples in quick succession;
without this guard each sample triggers a 47-byte write and the
firmware can't keep up, causing visible flicker.
2. Gate reapply_app_volumes on a 200 ms knob-quiet period. Calling
playerctl/pulsectl while the user is actively turning a knob stalls
the main loop, backing up serial data, missing heartbeats, and
causing the device LEDs to time out mid-turn.
Both state variables (last_led_colors, last_knob_event) are mutable
lists initialised in main() and threaded through handle_knob, keeping
module-level globals out of the picture.
Adds 6 new unit tests (56 total, all passing).
d69d94eaa2