Show maximum G on the running track-session screen

[PurpleSentinel]

Parent: #48

Summary

Show the maximum acceleration recorded during the current track session on the running-session screen, so the driver can see the session peak without leaving Timer Mode.

User experience

• Add a centred MAX <value>g readout above the countdown timer while a track session is running.
• Render it at native font size 2 in the upper safe area, with a clear visual gap above the countdown.
• Use the same dynamically selected high-contrast text colour as the countdown and elapsed timer.
• Reset the maximum at the start of every track session and retain the peak while the session is overrunning.
• Do not show the readout during rest sessions.
• If acceleration data is unavailable, show MAX -- and keep the timer and stop gesture fully operational.

Functional behavior

• Reuse G Mode's baseline-corrected, filtered planar acceleration magnitude so the displayed value has the same meaning in both modes.
• Calibrate the stationary baseline before launch detection/the session timer starts.
• Sample the shared IMU at a bounded interval without coupling input polling to display refreshes.
• Keep only the current session peak in memory; raw traces, persistence, summaries, and export remain in the scope of Capture IMU performance metrics and peak acceleration #48.
• Treat an IMU initialization, calibration, or read failure as a degraded display state rather than aborting a session that does not require launch detection.

Hardware, safety, and compatibility

• Reuse the existing QMI8658 instance; do not initialize competing drivers for the same device.
• Preserve launch detection, Auto rotation, Timer Mode stop handling, and the existing round-screen safe area.
• Avoid unbounded allocations in the live loop and preserve the current bounded input-polling cadence.

Acceptance criteria

• A running track session shows a centred maximum-G readout above the countdown.
• The readout uses font size 2 and is positioned higher, leaving a clear visual gap above the countdown.
• The peak starts from the calibrated session baseline, only increases during a session, and resets for the next track session.
• The peak remains visible during overrun and uses readable contrast throughout the green-to-red gradient and purple overrun background.
• Rest sessions do not show maximum G.
• Missing or failed IMU data shows MAX -- without blocking timing or the stop gesture when launch detection is disabled.
• Sampling remains bounded and does not make touch handling visibly less responsive.
• Host tests cover peak calculation, reset/degraded behavior, layout arguments, and track/rest frame behavior.
• The firmware build completes successfully.

Documentation

Update the README running-session description and screen layout to explain the new maximum-G readout and unavailable-data placeholder.

Relationship

This is a focused sub-issue of #48. That parent retains the broader work for stored metrics, session summaries, memory measurement, and exports.