locks: add process-wide lock hold time tracking

TrackedMutex and TrackedRwLock wrappers that record hold durations
by source location using #[track_caller]. Stats written to
~/.consciousness/lock-stats.json every second, sorted by max hold time.

Re-exported as crate::Mutex so all locks are instrumented. To disable,
swap the re-export back to tokio::sync::Mutex.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/hippocampus/store/persist.rs

@@ -21,16 +21,16 @@ use std::path::Path;
 use std::sync::Arc;
 
 /// Process-global cached store. Reloads only when log files change.
-static CACHED_STORE: tokio::sync::OnceCell<Arc<tokio::sync::Mutex<Store>>> =
+static CACHED_STORE: tokio::sync::OnceCell<Arc<crate::Mutex<Store>>> =
     tokio::sync::OnceCell::const_new();
 
 impl Store {
     /// Get or create the process-global cached store.
     /// Reloads from disk if log files have changed since last load.
-    pub async fn cached() -> Result<Arc<tokio::sync::Mutex<Store>>, String> {
+    pub async fn cached() -> Result<Arc<crate::Mutex<Store>>, String> {
         let store = CACHED_STORE.get_or_try_init(|| async {
             let s = Store::load()?;
-            Ok::<_, String>(Arc::new(tokio::sync::Mutex::new(s)))
+            Ok::<_, String>(Arc::new(crate::Mutex::new(s)))
         }).await?;
         {
             let mut guard = store.lock().await;