isInitializedFast

Internal flag to avoid AtomicBoolean.get overhead on every hot-path call.

Thread Safety: marked @Volatile so that the true written by the background coroutine in init is immediately visible to all other threads without requiring a full memory barrier on every read. Without @Volatile a thread that reads this field on a different CPU core may observe stale false indefinitely (data race / visibility bug on ARM64 weak memory model).