Function with_observer_thread_local 

pub fn with_observer_thread_local(o: Arc<dyn Observer>) -> ThreadObserverGuard

Sync RAII guard. Sets the per-thread observer override; restores the previous value on drop. Do not hold across .await — see the warning in spec 11 § 3.1; use Future::with_observer (Phase 3 task 3.3) for async.

Bumps OVERRIDE_COUNT on first install in this process.