Unit tests for depth-based home row mod logic

[flwrenn]

Summary

Comprehensive ztest suite for the home row mod module using synthetic ADC data and simulated timing.

Area

Firmware

Done when

• tests/home_row_mod/ test suite exists and passes on native_sim
• Tests cover: fast shallow tap -> TAP, deep press -> immediate HOLD, slow shallow hold -> delayed HOLD, normal typing speed (index vs pinky), rapid tap-hold-tap alternation, edge cases at exact thresholds
• Per-finger threshold configurations tested (pinky vs index finger sensitivity)
• False activation rate < 1/500 (matches S4 pass criterion)
• HOLD activation speed < 50ms for depth-triggered path (matches S4 pass criterion)

Dependencies

Depends on #53 (test infra), #57 (HRM module)

Context

These tests validate the HRM algorithm before hardware. The S4 pass criteria (HRM false activations: <1/500, activation speed: <50ms) should be test assertions. When Stage 4 begins, the algorithm should be proven correct so testing can focus on real-world feel tuning.