fix: #101 guard atomic shutdown hook with locked flag

[bibonix]

Fixes the spurious unlock at JVM shutdown described in #101. When the shutdown hook is installed before the acquisition loop reports success, a JVM exit during the wait — or an exception thrown by lock() or sleep() before this.locked.set(true) — used to call this.lock.unlock(this.label) for a lock that was never held; with the default empty label that release could match another holder on the server.

The fix in src/main/java/co/stateful/Atomic.java guards the hook body with an early if (!this.locked.get()) return, so the hook short-circuits whenever the acquisition step did not flip the flag. Successful call() paths still unlock through the existing finally block and remove the hook there, so behaviour for held locks is unchanged.

Ran mvn --batch-mode --errors clean install -Pqulice -DskipITs=true against the change: build succeeded, every unit test under src/test/java/co/stateful passed, and Qulice's quality gate completed without new findings.

Closes #101