[Bug] Avatar teeth render dark/gray instead of white (Unity parity)

### Platform
Mobile and Desktop (all Godot platforms)

### App Version
Not specified

### Device Information
Not specified

### Issue Description
Avatar teeth are rendered with incorrect material properties in Godot compared to Unity client. When avatars display facial expressions showing teeth, the teeth appear dark/grayish in Godot while they render as bright white in Unity.

### Expected Behavior
Teeth should render as bright white, matching the appearance in the Unity client.

### Screenshots / Media
See attached comparison image showing Unity (left) vs Godot (right) rendering of the same avatar with facial expression.

### Steps to Reproduce
1. Load an avatar with a facial expression that shows teeth
2. Compare the teeth color/material between Unity and Godot clients
3. Observe that Godot renders teeth much darker than Unity

### Occurrence
100% — consistent difference in all avatars with visible teeth

### Reference image

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Requested by Mateo via Slack