Add manual lap and split timing

[PurpleSentinel]

Parent: #23

Summary

Record lap or split markers during a track session and show useful current, previous, and best values.

Acceptance criteria

• Choose a gesture or external-event path that cannot be confused with stop.
• Record monotonic split timestamps without delaying the countdown loop.
• Show the last and best lap using a driver-readable layout.
• Handle accidental double events with configurable debounce.
• Expose recorded laps to the session summary and export features.

Roadmap group

Session workflow and timing