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>

crates/core/Cargo.toml

@@ -11,3 +11,6 @@ thiserror = "2"
 sysinfo = "0.35"
 async-trait = "0.1"
 num_cpus = "1"
+
+[dev-dependencies]
+tempfile = "3"