Add circular session progress display

[PurpleSentinel]

Summary

• add a thin 36-segment circular progress ring to active track and rest sessions
• add explicit TRACK, WARNING, FINAL LAP, OVERRUN, and REST phase labels
• refine the running MAX-G hierarchy with a smaller MAX caption and larger tabular value/unit
• keep the countdown layout and once-per-visible-frame refresh policy unchanged
• lazy-load the new Timer-only graphics so the RP2040 retains its startup memory margin
• update the README, user guide, and regression coverage

Why

The round screen can communicate scheduled progress without displacing the primary countdown, while phase labels ensure state does not rely on colour alone. Physical testing also exposed a startup MemoryError when the drawing code lived in the touch driver; isolating it from the startup path restores reliable boot behaviour.

Validation

• PYTHONPATH=firmware python -m unittest discover -s tests — 211 tests pass
• full-ring render overhead measured at approximately 5.5 ms on the supported RP2040 device
• real device startup completed with approximately 54 KB free after startup imports
• delayed Timer graphics imported with approximately 27 KB free
• physical deployment, boot checks, and reset completed through Docker on /dev/ttyACM0 with user.json preserved

Closes #36