fix: Prevent startup rendering delay by waiting for first render state

[braedonsaunders]

The terrain map was rendering before buildings, units, and minerals for a few seconds at startup. This was caused by the loading screen completing before entity data arrived from the game worker.

Changes:

• Add waitForFirstRenderState() method to WorkerBridge that resolves when the first render state with entities is received from the worker
• Update WebGPUGameCanvas to await this method after spawning entities, ensuring all game objects are ready before the loading screen fades out and the countdown begins
• Make GameWorker send an initial render state immediately after spawning entities (before the game loop starts), so the main thread receives entity data during initialization

This integrates with the existing Phaser overlay countdown - entities are now guaranteed to be visible before the countdown starts.

https://claude.ai/code/session_013CXJ1mTpfxWUY7FHXT54gY