Fix configuration persistence and add host tests

[PurpleSentinel]

Summary

• make timer/session logic importable and testable on CPython
• preserve current editor selections and safely persist a launch sensitivity of zero
• validate, migrate, and atomically/recoverably store configuration
• establish canonical v3.2 configuration keys and update user documentation
• add a 23-test host suite and GitHub Actions workflow

Root cause

Configuration values were selected and updated using fixed indexes and truthiness checks, so opening an editor reset its value and SENSITIVITY=0 corrupted the in-memory settings object. JSON writes truncated the active file before serialization and unvalidated keys were injected into globals. Hardware imports also prevented normal host-side regression testing.

User impact

Track/rest selections now reopen at their saved values, Launch Mode can be disabled persistently, damaged or legacy configuration is recovered into safe canonical values, and future changes receive automated host-side coverage.

Physical LCD, touch, IMU, pinout, and real-world launch threshold behavior remain outside host test coverage.

Validation

• python3 -m unittest discover -s tests -q — 23 tests passed
• mocked firmware entry-point import
• Python AST and JSON parsing
• git diff --check

Closes #6
Closes #7
Closes #8
Closes #9
Closes #12