Add session maximum G display

[PurpleSentinel]

Summary

• show a baseline-corrected maximum planar G value above the running track-session countdown
• share filtering and calibration logic with graphical G Mode and reuse the same baseline for Launch Mode
• keep live sampling bounded without increasing normal framebuffer redraw frequency
• degrade to MAX -- if the IMU is unavailable while preserving timer and stop controls
• document the calibration step, display behavior, and firmware upload manifest

User impact

Drivers can see the maximum acceleration reached during the current track session without leaving Timer Mode. The compact, clearly spaced MAX 1.23 g line resets for each session, remains visible through overrun, and does not appear during rest sessions.

Validation

• python -m compileall -q .
• python -m unittest discover -s tests -q (152 tests)
• git diff --check
• QMI8658 hardware smoke test on the connected Waveshare RP2040 device
• firmware upload and successful normal boot on MicroPython 1.21.0

Closes#69