Adds the public probe entry point with two override mechanisms:
- In-process TEST_OVERRIDE slot serialised by TEST_LOCK mutex for
unit tests (avoids races with cargo's parallel runner).
- MAGNOTIA_POWER_STATE_OVERRIDE env var for integration tests set
externally.
Platform dispatch: Linux reads /sys/class/power_supply; all others
return Unknown. Five new override tests pass alongside the six sysfs
parser tests from Task 1.2 (12 total green).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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