Throttle live-screen redraws to timer changes

[PurpleSentinel]

Problem

The track and rest session loops have no delay or change-detection guard. Every iteration calls Touch.LiveScreen(), which clears and transfers the complete 240×240 RGB565 framebuffer even though the displayed timer generally changes only once per second.

This wastes CPU and power, repeatedly reconfigures the touch controller, and increases SPI/I²C contention during the most important runtime state.

Relevant locations

• main.py, track-session loop
• main.py, rest-session loop
• touch_drive.py, LiveScreen
• lcd_1inch28.py, show

Acceptance criteria

• Redraw only when the displayed second, warning state, or another visible value changes.
• Add a bounded loop delay or use a monotonic scheduling approach.
• Continue checking stop/clear gestures responsively between redraws.
• Avoid rewriting touch-controller mode on every frame when the mode is unchanged.
• Measure and document expected refresh/input responsiveness on hardware.
• Add a test confirming redraw count remains bounded for a simulated session.

Resolution

Resolved by #17.

• 48 host tests pass.
• A simulated six-second session produces exactly six redraws.
• A physical three-second MicroPython session produced exactly three redraws and 49 input checks.
• Full-frame hardware redraws measured 56.2–65.4 ms.
• Input is checked within 50 ms normally and approximately 115 ms worst-case during a redraw.
• Five consecutive hardware frames produced only the two initial gesture-mode register writes.

[PurpleSentinel]

Resolved and hardware-verified by #17 (25aecfc).

The live session loop now polls gestures every 50 ms but transfers the framebuffer only when a displayed second, warning state, or another visible value changes. The CST816T mode is cached, eliminating repeated mode writes.

Validation: 48 host tests passed; a real three-second session produced three redraws and 49 input checks; full redraws measured 56.2–65.4 ms; worst-case input-check latency measured approximately 115 ms. The connected timer is running the merged implementation.