Add persistent average lap-time setting

[PurpleSentinel]

Summary

• add Avg Lap Time beneath Auto-Dim in the Settings carousel
• provide separate minute and second editors with the standard left/right, swipe-up, and swipe-down controls
• persist one validated AVG_LAP_TIME_SECONDS value from 00:00 through 59:59
• migrate existing settings safely and include the unset 00:00 value in Restore defaults
• document the value's reserved-for-future-lap-features scope

User impact

Drivers can save the expected average lap duration for the current circuit. 00:00 means not configured. This increment stores the setting only and does not change current session timing, G Mode, the Ready screen, or session review.

Compatibility

Older user.json files gain the new zero default without losing existing preferences. Invalid types or out-of-range values are normalized safely, and previewing or cancelling the editor does not write to flash.

Validation

• python3 -m compileall -q firmware tests tools
• python3 -m unittest discover -s tests -t . — 195 tests pass
• verified both editor layouts fit the 240x240 circular safe area
• deployed to the connected supported Waveshare device while preserving its settings
• confirmed on-device migration, 01:02 formatting, real LCD rendering, and clean Timer Mode boot

Closes #77