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",
 ]
 

crates/core/Cargo.toml

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

crates/core/src/lib.rs

@@ -5,6 +5,7 @@ pub mod model_registry;
 pub mod paths;
 pub mod process_watch;
 pub mod recommendation;
+pub mod power;
 pub mod types;
 
 pub use error::{MagnotiaError, Result};

crates/core/src/power.rs

@@ -0,0 +1,27 @@
+//! Power-state probe for inference thread tuning.
+//!
+//! Reports whether the machine is running on AC or battery so callers
+//! can drop thread counts when energy matters more than throughput.
+//! Linux uses the documented sysfs ABI under
+//! `/sys/class/power_supply/`. macOS and Windows return `Unknown` for
+//! now; native probes (IOPSGetProvidingPowerSourceType,
+//! GetSystemPowerStatus) are deferred.
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum PowerState {
+    OnAc,
+    OnBattery,
+    Unknown,
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn power_state_variants_are_distinct() {
+        assert_ne!(PowerState::OnAc, PowerState::OnBattery);
+        assert_ne!(PowerState::OnAc, PowerState::Unknown);
+        assert_ne!(PowerState::OnBattery, PowerState::Unknown);
+    }
+}