fix(core): serialise tuning env-var tests behind a lock

Without a lock, env_var_bypasses_clamps (set_var + remove_var) and
matches_existing_clamp_when_no_clamps_apply (remove_var) race under
cargo's parallel test runner. Task 2.2 will add another remove_var
call, compounding the race.

Adds a #[cfg(test)] static THREAD_ENV_LOCK and a closure-style
with_thread_env_lock helper, mirroring power::with_override.

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

crates/core/src/tuning.rs

@@ -63,24 +63,38 @@ pub fn inference_thread_count(_workload: Workload, _gpu_offloaded: bool) -> usiz
 mod tests {
     use super::*;
 
+    /// Serialises tests that read/write `MAGNOTIA_INFERENCE_THREADS` so
+    /// they don't race under cargo's parallel test runner.
+    /// Mirrors the pattern used by `power::with_override`.
+    fn with_thread_env_lock<R>(body: impl FnOnce() -> R) -> R {
+        use std::sync::Mutex;
+        static THREAD_ENV_LOCK: Mutex<()> = Mutex::new(());
+        let _lock = THREAD_ENV_LOCK.lock().expect("tuning thread-env lock poisoned");
+        body()
+    }
+
     #[test]
     fn matches_existing_clamp_when_no_clamps_apply() {
         // With no env override, no battery, no gpu_offload, helper
         // should return physical-core count clamped to [2, 8].
         // We can't pin physical exactly without mocking num_cpus; just
         // assert the result is in range.
+        with_thread_env_lock(|| {
             std::env::remove_var("MAGNOTIA_INFERENCE_THREADS");
             let n = inference_thread_count(Workload::Llm, false);
             assert!(n >= MIN_INFERENCE_THREADS && n <= MAX_INFERENCE_THREADS,
                     "got {n}, expected within [{MIN_INFERENCE_THREADS}, {MAX_INFERENCE_THREADS}]");
+        });
     }
 
     #[test]
     fn env_var_bypasses_clamps() {
+        with_thread_env_lock(|| {
             std::env::set_var("MAGNOTIA_INFERENCE_THREADS", "10");
             let n = inference_thread_count(Workload::Llm, true);
             assert_eq!(n, 10);
             std::env::remove_var("MAGNOTIA_INFERENCE_THREADS");
+        });
     }
 
     #[test]