fix(multi-client): Prevent duplicate lights when multiple clients connect

[simplast]

Problem Summary:

Issue: Light intensity accumulation with multiple client connections

Description: When multiple web clients connect to the same gzbridge instance, the existing client's scene becomes progressively brighter with each new client connection.

Symptoms:

• Client A connects normally with correct lighting
• When Client B connects, Client A's scene becomes overbright
• With Client C connection, lighting becomes even brighter
• Each additional client connection adds to the light accumulation

Root Cause: The initScene() function in gzscene.js creates a new sun light on every client initialization without checking if lights already exist, causing duplicate light sources to accumulate in the shared THREE.js scene.

Impact: Visual degradation where scenes become unnaturally bright as more clients connect, making the simulation unusable for existing clients.

Reproduction:

1. Open first web client → lighting appears normal
2. Open second web client → first client's scene becomes brighter
3. Open third web client → lighting becomes even more overbright
4. Repeat → progressive light accumulation