Pure function takes a directory path and returns OnAc, OnBattery, or
Unknown by reading `type` and `online` files in each supply entry.
Matches the kernel ABI documented at
Documentation/ABI/testing/sysfs-class-power. Failure modes (missing
dir, unreadable files, malformed contents) all fall through to Unknown
rather than panicking.
Seven tests cover: Mains online, battery-only, USB-PD, empty dir,
missing dir, and malformed entries. All pass.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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