Add continuous automatic display orientation

[PurpleSentinel]

Summary

• add an Auto rotation choice alongside the existing fixed orientations
• use filtered QMI8658 gravity data to keep the display upright while the device rotates
• rotate LCD output and touch coordinates together without interrupting an active timer
• share the IMU between Auto rotation, G mode, and Launch mode, with graceful fallback if it is unavailable
• update configuration guidance, deployment documentation, and automated coverage

Why

Drivers can mount or rotate the device in any supported orientation while keeping every screen and touch target upright. Fixed rotation remains available for installations that do not want sensor-driven orientation.

Hardware and memory handling

Connected-device validation exposed heap fragmentation when battery.py was first imported after Auto rotation had allocated its filter state, despite roughly 61 KB of total free memory. Battery support is now preloaded before filtered sampling begins and retained across Timer-mode transitions, avoiding the allocation failure without adding another framebuffer.

Impact

• existing numeric DISPLAY_ROTATION_DEG values remain compatible
• selecting Auto stores "auto" in the existing setting
• ambiguous motion or a near-horizontal device retains the last stable orientation
• sensor failure freezes the last orientation while fixed rotation and timer operation remain available

Validation

• python -m unittest discover -s tests — 136 tests passed
• fixed-orientation and Auto startup paths tested on the connected Waveshare RP2040 under MicroPython 1.21.0
• two Auto cold-start runs completed both splash screens and reached Timer Mode without a traceback
• original on-device user settings restored after validation

Closes#64