Support an external GPIO start, lap, or stop trigger

[PurpleSentinel]

Parent: #23

Summary

Allow a wheel button, switch, transponder output, or other isolated accessory to control timing without touching the screen.

Acceptance criteria

• Provide a configurable action and input polarity with debounce.
• Document safe voltage levels, grounding, and isolation requirements.
• Resolve conflicts between touch and external events deterministically.
• Degrade normally when the input is absent.
• Add mocked edge, bounce, and simultaneous-event tests.

Roadmap group

Sensors, power, and hardware expansion