add double-blink detection to toggle head-tracking on/off

Uses Vision framework face landmarks to compute Eye Aspect Ratio (EAR)
from eye contour bounding boxes. A state machine detects individual blinks
(closed→open transition with hysteresis) and fires a double-blink event
when two blinks occur within 0.6s. Double-blink toggles monitor focus
switching — camera stays active so blinks are always detected.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

v0.9.3

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

v0.9.2

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

v0.9.1

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

v0.9.0

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>