[WIP] Add device orientation calibration mechanism

[adam-weber]

Summary

• Add mount calibration system for determining device orientation relative to vehicle using brake-based yaw offset detection
• Add NVS (Non-Volatile Storage) persistence so calibration survives device reboots
• Fix speed source for calibration: use EKF velocity (200 Hz) instead of stale GPS speed (5 Hz)

Details

The device can be mounted in any horizontal orientation. The calibration system determines the yaw offset between the device's forward axis and the vehicle's forward axis using a simple brake calibration maneuver.

When braking hard in a straight line, the acceleration vector points directly backward relative to the vehicle. The angle of this vector in the device's body frame is the mounting offset.

State machine flow

1. User triggers calibration via web UI
2. System waits for vehicle to reach minimum speed (3 m/s)
3. System waits for hard braking (>0.25g) while driving straight (<8.6 deg/s yaw
   rate)
4. Collects 0.5s of acceleration samples
5. Computes mean acceleration angle, adds 180 degrees to get mount offset
6. Validates sample consistency (<10 degree variance)

NVS Persistence

New nvs_storage.rs module provides:

• Generic typed getters/setters (u8, i16, f32)
• Convenience methods for mount calibration (load/save)
• Write ordering ensures safety against power loss: data written before validity flag.

Speed Source Fix

Previously the calibrator used GPS speed which updates at 5 Hz and can be stale. Now uses EKF velocity estimate which updates at IMU rate (200 Hz) for responsive state transitions during the calibration maneuver.

[adam-weber]

I'll test this in a car later