[fx] Optimize composite sprite cache invalidation

[S1mplector]

Module: FX / Renderer

Goal:
Avoid unnecessary recompositing of layered character sprites during rapid expression and timeline updates.

Acceptance criteria:

• Cache keys account for only the state that changes rendered output.
• Unchanged layered expressions are reused across frames.
• Metrics or tests demonstrate fewer recomposites in a rapid expression swap scenario.
• No stale image artifacts appear after expression, layer proxy, or transform updates.

Notes:
Keep the cache invalidation rules easy to audit; correctness beats aggressive caching.