Throttle live display redraws

[PurpleSentinel]

Summary

• compare the complete visible live-session frame and redraw only when its displayed second, warning state, or another visible value changes
• poll stop and clear gestures every 50 ms between redraws
• share one throttled loop across track and rest sessions
• cache the CST816T mode so unchanged frames do not rewrite touch-controller registers
• document measured display and input responsiveness and include the new module in installation instructions

Root cause

The track and rest loops previously cleared and transferred the complete 115,200-byte framebuffer on every unbounded iteration. They also called Set_Mode() for every live frame, producing needless SPI traffic, I2C writes, CPU use, and contention even though the visible timers generally change once per second.

Validation

• python3 -m unittest discover -s tests -v — 48 tests passed
• a simulated six-second session produced exactly six redraws while input remained continuously polled
• warning-state changes redraw even when the visible time strings are unchanged
• repeated unchanged touch modes produce no additional register writes
• on the connected Waveshare board, five full redraws measured 56.2–65.4 ms
• measured input polling is 50 ms normally and approximately 115 ms worst-case during a redraw
• a real three-second MicroPython session produced exactly three redraws and 49 input checks
• five hardware frames produced only the two initial gesture-mode register writes

Fixes #10