Capture IMU performance metrics and peak acceleration

[PurpleSentinel]

Parent: #23

Summary

Record bounded acceleration metrics during a session for driver review and launch diagnostics.

Acceptance criteria

• Choose a sampling rate that does not degrade display or touch responsiveness.
• Report peak magnitude and configurable summary metrics rather than unbounded raw data by default.
• Handle IMU loss by degrading normal timing cleanly.
• Include metrics in summaries and exports when available.
• Measure memory, flash, and CPU cost on hardware.

Roadmap group

Sensors, power, and hardware expansion