Add a circular progress indicator and explicit phase labels

[PurpleSentinel]

Parent: #23

Summary

Use the round display to show session progress and clearly identify track, warning, overtime, and rest phases.

Acceptance criteria

• Keep the countdown as the primary visual element.
• Update progress no more often than needed for smooth, efficient rendering.
• Use labels or symbols so phase changes do not depend only on colour.
• Guarantee no clipping around the circular edge.
• Measure redraw cost and input responsiveness on hardware.

Implementation

Implemented by #82 with a 36-segment, two-pixel perimeter ring, explicit phase labels, and a lazily loaded Timer-only graphics module that preserves the RP2040 startup memory margin.

Roadmap group

Driver display and alerts

[PurpleSentinel]

Development has started on the isolated local branch feature/issue-36-session-progress-ring as a deliberately reversible prototype.

Current prototype behavior:

• a two-pixel, 36-segment high-contrast perimeter arc empties in proportion to scheduled track and rest time;
• the primary countdown, MAX-G value and lower display retain their existing positions and sizes;
• explicit labels show TRACK, final-third WARNING, OVERRUN, and REST;
• FINAL LAP is used only when the user has selected LAPS LEFT and the average-based estimate reaches one lap or less;
• the existing once-per-visible-frame refresh policy is retained.

Validation so far:

• 211 hardware-independent tests pass;
• geometry tests keep every arc point within the 240 × 240 circular panel and keep the longest label clear of MAX-G;
• measured on the supported RP2040-Touch-LCD-1.28-B: a worst-case full ring adds about 5.5 ms of framebuffer work; the measured warm full frame was approximately 80.7 ms render plus 19.1 ms LCD transfer;
• deployed to the connected reference device through the Docker device-access workflow with its existing user.json preserved.

The prototype has intentionally not been committed, pushed, or merged pending physical/UI assessment.

[PurpleSentinel]

Implemented and merged in #82.

Final behaviour:

• a two-pixel, 36-segment perimeter ring empties proportionally during track and rest sessions;
• TRACK, WARNING, conditional FINAL LAP, OVERRUN, and REST labels provide non-colour state cues;
• the primary countdown remains fixed and dominant;
• the MAX-G line now uses a smaller MAX caption with a larger tabular value and unit;
• Timer-only drawing code is lazy-loaded so it does not consume the RP2040's startup heap.

Validation completed:

• 211 host tests passed locally and both GitHub Actions runs passed;
• the full ring added approximately 5.5 ms of framebuffer work on the reference device;
• after correcting the startup module lifetime, the real device booted with approximately 54 KB free after startup imports and loaded the Timer graphics with approximately 27 KB free;
• the final firmware was deployed and reset through the required Docker /dev/ttyACM0 workflow with device settings preserved.