feat: add sprite workflow for portraits

[oco777]

Summary

• add PortraitSprite and Sprites fields to FUnityActorData and expose sprite metadata through ActorState/ActorPresenter
• update ActorPresenter, ActorView, and FUnityManager to prefer sprites with Texture2D fallback plus sprite index APIs for Visual Scripting
• document the new sprite workflow across AGENTS.md, CONTRIBUTING.md, and VS_Scratch_Mapping.md and add changelog entries

Testing

• not run (Unity project)

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Codex Task