Add optional GPS lap timing integration

[PurpleSentinel]

Parent: #23

Summary

Use a supported external GPS receiver to identify start-finish crossings and calculate lap times.

Acceptance criteria

• Define supported receiver protocol, baud rate, wiring, and update rate.
• Allow start-finish location capture and later reuse.
• Use direction and hysteresis to prevent duplicate crossings.
• Continue the normal session timer when GPS is unavailable.
• Validate with recorded GPS traces before track testing.

Roadmap group

Sensors, power, and hardware expansion