Add optional audible and haptic alerts

[PurpleSentinel]

Parent: #23

Summary

Support external buzzer or vibration hardware for start, warning, expiry, and pit-rest events.

Acceptance criteria

• Keep the feature disabled when no alert hardware is configured.
• Allow individual event alerts and intensity or pattern settings.
• Use non-blocking output so timing and touch remain responsive.
• Document wiring and electrical constraints for supported accessories.
• Provide mocked timing tests and physical hardware validation.

Roadmap group

Driver display and alerts