Manager.mjs: Refactor onWorkerMessage to remove redundancy

The onWorkerMessage method in src/worker/Manager.mjs has accumulated redundant logic for handling message replies, specifically the promise registration and sendMessage sequence. This has been exacerbated by the recent addition of updateVdom handling and the zero-delta optimization.

Goal: Refactor onWorkerMessage to eliminate code duplication by extracting the common reply/promise logic into a reusable helper method.

Plan:

1. Identify the common pattern:
   • Check for deltas.
   • If empty -> Immediate reply.
   • If present -> Register promise -> Fire event -> Send reply on promise resolution.
2. Create a helper method handleDomUpdate({ data, eventName }).
   • data: The message payload.
   • eventName: The event to fire (e.g., 'updateVdom', 'automount').
3. Update onWorkerMessage to use this helper for both action === 'updateVdom' and the action === 'reply' (VDOM) path.

This will reduce file size and improve maintainability.