Add ZUPT/ZARU pseudo-measurements with stationary detection

[jbrodovsky]

Context

Zero-velocity (ZUPT) and zero-angular-rate (ZARU) updates are standard in strapdown systems and improve bias stability and drift during stationary periods.

Scope / Requirements

• Implement stationary detection based on accel/gyro norms over a sliding window.
• Add measurement models for zero velocity and zero angular rate.
• Allow thresholds and window length to be configured in the simulator.
• Log detected stationary intervals for debugging.

Acceptance Criteria

• On datasets with stationary segments, ZUPT/ZARU updates reduce drift and keep biases bounded.
• Unit tests confirm detector triggers on static synthetic data and stays inactive on dynamic data.
• Documentation explains thresholds and units.