feat(core): add PowerState skeleton in new power module

Introduces crates/core/src/power.rs with the PowerState enum
(OnAc / OnBattery / Unknown) and a unit test confirming the three
variants are distinct. Registers the module in lib.rs and adds
tempfile = "3" to dev-dependencies for use in later tasks.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -2859,6 +2859,7 @@ dependencies = [
  "serde",
  "serde_json",
  "sysinfo",
+ "tempfile",
  "thiserror 2.0.18",
 ]