Add continuous automatic display orientation

[PurpleSentinel]

Parent: #23

Summary

Extend the four fixed mounting orientations delivered by #33 with a new Auto option. Auto orientation should use the onboard QMI8658 accelerometer's gravity vector to keep the display upright and directional gestures intuitive as the device is rotated, updating without a restart or a visit back to Settings.

User experience

• Add Auto to the existing Settings → Rotation choices alongside 0°, 90°, 180°, and 270°.
• Preview Auto immediately. The preview should show both Auto and the currently detected quarter-turn orientation so the user can verify it before saving.
• Swipe up to save Auto; swipe down to cancel and restore the prior manual or automatic setting.
• When Auto is saved, startup splashes and subsequent screens should adopt the detected upright orientation as soon as a reliable gravity reading is available.
• Continue updating while the application is running, including Ready, configuration, Timer, rest, Launch, G Mode, menus, and error screens. An orientation change must not reset, pause, or otherwise alter an active session.
• Change touch-direction mapping at the same time as the display, so prompts remain relative to the text currently shown.
• Keep all existing fixed-angle choices and their behavior unchanged.

Functional behavior

Detection

• Use filtered QMI8658 acceleration rather than touch coordinates or elapsed motion.
• Project gravity onto the display plane and classify only the four supported quarter-turn orientations.
• Apply dominance thresholds, hysteresis, and a short stability period before changing orientation so road vibration, acceleration, and positions close to a 45° boundary do not cause rapid flipping.
• Redraw the current framebuffer after a confirmed change; do not allocate a second 240×240 framebuffer.
• Do not write each detected angle to flash. Persist only that the user's selection is Auto, retaining the detected angle in RAM.
• When the display is too close to horizontal for gravity to reveal rotation about the screen normal, retain the last reliable angle. This is a physical accelerometer limitation, not an error.

Configuration and compatibility

• Persist Auto in the canonical user configuration while continuing to accept existing integer DISPLAY_ROTATION_DEG values.
• Existing user.json files and all fixed-angle selections must load without change.
• Restore Defaults should continue to select fixed 0° rotation.
• Auto must cause the IMU to initialize even when Launch Mode is disabled, while sharing the existing QMI8658 instance with Launch Mode and G Mode.

Degraded behavior

• If the QMI8658 is unavailable at startup, keep Timer Mode usable and retain a safe fixed orientation rather than repeatedly changing the screen.
• The Auto preview must clearly report that automatic detection is unavailable and allow cancel/back navigation.
• If sampling fails after startup, freeze at the last reliable angle, log/show an actionable degraded-state message at an appropriate safe point, and preserve normal timing.
• A missing IMU must not break existing manual rotation choices.

Hardware, safety, and compatibility

• Target the supported Waveshare RP2040-Touch-LCD-1.28 and its onboard QMI8658, GC9A01A, and CST816S.
• Keep orientation polling bounded and responsive without increasing framebuffer memory use or materially delaying touch/session loops.
• Coordinate IMU access with Launch and G Mode so there are no duplicate sensor objects or conflicting reads.
• Never interrupt or restart an active track/rest session solely because orientation changes or automatic sensing degrades.
• Verify the native-axis/sign mapping on physical hardware at all four quarter turns.

Acceptance criteria

• Rotation Settings offers Auto plus all four existing fixed angles.
• Auto preview reports the live detected angle and supports save/cancel safely.
• Auto persists across restart without replacing the setting with a detected integer angle.
• The screen redraws upright and touch directions remain intuitive after each stable quarter-turn change.
• Detection uses hysteresis/stability filtering and does not oscillate at boundaries or under short acceleration spikes.
• A near-horizontal device retains its last reliable orientation.
• Active track/rest timing continues uninterrupted through orientation changes.
• Existing fixed rotation, Launch Mode, G Mode, brightness, restore-default, and configuration behavior remain compatible.
• Missing or failing IMU behavior is safe, actionable, and does not block Timer Mode or manual rotation.
• Host tests cover four-way detection, filtering, hysteresis, stability, flat-device retention, persistence/migration, preview/save/cancel, degraded IMU behavior, gesture mapping, and circular layout.
• The complete suite passes and Auto is exercised on the supported physical device at all four quarter turns.

Documentation

• Update README and User Guide with Auto selection, live behavior, the near-horizontal limitation, degraded behavior, and the required deployment module list.

Relationship

This is a follow-up enhancement to #33 and the v4.1.0 fixed-orientation release. It remains part of roadmap epic #23.

[PurpleSentinel]

Implemented and validated in #65, merged into the v4.1 branch.