[fx] Add pixel regression coverage for expression crossfades

[S1mplector]

Module: FX / Renderer

Goal:
Verify expression transitions render smoothly without blank or missing character layers.

Acceptance criteria:

• Test scene renders old expression, transition midpoint, and final expression.
• Pixel checks confirm expected layers are present at each stage.
• Test covers layered characters with mouth/face changes.
• Regression fails if transition briefly renders a blank head or missing mouth.

Notes:
Use deterministic test assets or existing lightweight fixtures.