fix(worker): register leader-now listener early to prevent race condition

[kaigritun]

Summary

Fixes #850

Problem

There is a race condition in PGliteWorker leader election when the main thread is under heavy load (e.g., during React hydration or complex app initialization).

The leader-now message sent by the worker immediately after ready could be lost because the main thread was still blocked or had not yet attached the message event listener.

This resulted in isLeader remaining false indefinitely, even though the worker successfully acquired the lock and was acting as the leader.

Root Cause

The listener for leader-now was registered after several async operations:

1. await acquireLock(tabCloseLockId) - yields the main thread
2. BroadcastChannel setup
3. Tab channel setup

During this time, the worker could send leader-now before the listener was in place.

Fix

Move the leader-now listener registration to immediately after workerReadyPromise resolves, before any async operations that could yield control.

This ensures the listener is in place before the worker has any chance to send the message.

Testing

• The fix is logically sound: the listener must be registered before any await that could allow the worker to send the message
• The original issue reporter provided detailed logs showing the race window was only ~0.4ms, which this fix closes by not having any async gap at all