Recover an active session after unexpected power loss

[PurpleSentinel]

Parent: #23

Summary

Offer best-effort recovery of an interrupted track or rest session when suitable timekeeping hardware is available.

Acceptance criteria

• Define behavior with and without an external RTC or retained monotonic time.
• Checkpoint state at a flash-safe frequency.
• Ask the user whether to resume, finish, or discard the recovered session.
• Never present guessed elapsed time as exact.
• Test resets at every session phase and corrupted checkpoints.

Roadmap group

Sensors, power, and hardware expansion