fix(core): cfg-gate test override + restore TEST_OVERRIDE on panic

TEST_OVERRIDE and test_get_override are now #[cfg(test)]-gated and
the read in probe_power_state is wrapped in a #[cfg(test)] block,
so test scaffolding doesn't compile into production builds.

with_override now uses a drop-guard pattern so a panic inside body
still resets TEST_OVERRIDE, preventing stale state from leaking
into subsequent tests.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/core/src/power.rs

@@ -64,6 +64,7 @@ fn read_trimmed(path: &Path) -> Option<String> {
     fs::read_to_string(path).ok().map(|s| s.trim().to_owned())
 }
 
+#[cfg(test)]
 use std::sync::Mutex;
 
 /// Top-level power-state probe.
@@ -80,6 +81,7 @@ use std::sync::Mutex;
 ///
 /// (TTL cache is added in Task 1.4.)
 pub fn probe_power_state() -> PowerState {
+    #[cfg(test)]
     if let Some(state) = test_get_override() {
         return state;
     }
@@ -111,14 +113,29 @@ fn platform_probe() -> PowerState {
 
 // In-process override slot. Tests must use `with_override` (below) to
 // set it; production code never writes to this. Read on every probe.
+#[cfg(test)]
 static TEST_OVERRIDE: Mutex<Option<PowerState>> = Mutex::new(None);
 
+#[cfg(test)]
 fn test_get_override() -> Option<PowerState> {
     *TEST_OVERRIDE.lock().expect("power test override mutex poisoned")
 }
 
+/// Drop-guard that resets `TEST_OVERRIDE` to `None` on drop (including on unwind).
+/// This prevents a panicking test body from leaking stale override state into
+/// subsequent tests.
+#[cfg(test)]
+struct OverrideGuard;
+
+#[cfg(test)]
+impl Drop for OverrideGuard {
+    fn drop(&mut self) {
+        *TEST_OVERRIDE.lock().expect("power test override mutex poisoned") = None;
+    }
+}
+
 /// Run `body` with the in-process override set to `state`. Restores
-/// the previous override (always `None` in practice) on return.
+/// `TEST_OVERRIDE` to `None` on return, even if `body` panics.
 ///
 /// Holds a dedicated `TEST_LOCK` mutex for the duration of the body,
 /// so override-using unit tests run serially with respect to each
@@ -128,9 +145,8 @@ pub(crate) fn with_override<R>(state: Option<PowerState>, body: impl FnOnce() ->
     static TEST_LOCK: Mutex<()> = Mutex::new(());
     let _lock = TEST_LOCK.lock().expect("power TEST_LOCK poisoned");
     *TEST_OVERRIDE.lock().expect("power test override mutex poisoned") = state;
-    let result = body();
-    *TEST_OVERRIDE.lock().expect("power test override mutex poisoned") = None;
-    result
+    let _guard = OverrideGuard;
+    body()
 }
 
 #[cfg(test)]