Record bounded crash and reset diagnostics

[PurpleSentinel]

Parent: #23

Summary

Retain enough information after an unexpected reset to identify failure stage and frequency without exhausting flash.

Acceptance criteria

• Record reset cause, firmware version, last application state, and a concise error.
• Use a bounded log with controlled write frequency.
• Show the latest fault in diagnostics and expose it over USB.
• Allow logs to be cleared deliberately.
• Test malformed logs, repeated resets, and storage limits.

Roadmap group

Data, configuration, and maintenance