Add estimated laps remaining display

[PurpleSentinel]

Summary

• add a conditional Lower Display setting when Avg Lap Time is configured
• let drivers choose the existing elapsed Count Up line or an estimated laps-remaining display
• derive laps directly from scheduled seconds remaining divided by the saved average lap time
• reset the choice atomically to Count Up when Avg Lap Time is cleared
• render a small LAP caption above a larger numeric estimate without moving or resizing the main countdown
• keep pit/rest sessions on the existing elapsed count-up display

User impact

Track sessions can now show an approximate remaining-lap count beneath the main countdown. Values below 100 laps use one decimal place, larger estimates use rounded whole numbers, and overrun shows 0.0. Missing or invalid average-lap data falls back safely to elapsed time.

Compatibility and safety

Existing installations migrate to Count Up. The option remains hidden until Avg Lap Time is non-zero, previews do not write to flash, and clearing the dependency cannot leave an unavailable selection behind. The maximum-G line, main countdown, gradient, contrast, polling cadence, rest screen, and review flow are unchanged.

Validation

• python3 -m compileall -q firmware tests tools
• python3 -m unittest discover -s tests -t . — 207 tests pass
• verified the small label and enlarged maximum 3600 value fit the 240x240 circular safe area without touching the main countdown
• deployed to the connected supported Waveshare device while preserving settings
• confirmed conditional Settings availability directly on MicroPython
• rendered LAP plus a larger 6.7 value on the physical display with the main 10:00 countdown unchanged
• rebooted successfully into normal Timer Mode

Closes #79