[fx] Crossfade only changed layers during expression swaps

[S1mplector]

Module: FX / Renderer

Goal:
Improve layered expression transitions by keeping unchanged layers stable and crossfading only layers that actually change.

Acceptance criteria:

• Expression diff identifies unchanged, removed, added, and changed layers.
• Unchanged layers remain stable during transition and are not flashed or redrawn as blank.
• Added/removed/changed layers transition smoothly according to duration.
• Tests cover mouth-only and face-only expression swaps.

Notes:
This should reduce visual flashing and avoid full-character flicker during simple expression changes.