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>
