TASK 31: SceneGraph component + baked-crust compositor optimization
- Add ElementKind (Static/Dynamic) to SceneElement base; Source/WebcamSceneConfig classify - Services/SceneGraph.cs: owns the Scenes collection (ViewModel's Scenes delegates to it), the element mutation surface (Add/Insert/Remove/Move, each invalidating the bake), and the queries that were scattered LINQ (GetBackground/GetWebcam/GetChatBoxes/GetSplitPoint/IsStatic) - SceneCompositor: split-aware BakeStaticBase + CompositeLayers + Render(.., staticBase, split); builds/caches the static base below the split point in source-rect space - FramePump: optional SceneGraph -> optimized bake+composite path; falls back to full render - MainViewModel: routes element mutations through the graph; invalidates the bake on static layout/opacity/visibility/useDefaultBackground changes and after background heal/Ensure - Integration test SceneGraphTests.BakedStaticBase_WithDynamicLayer_CompositesCorrectly Derivative survey (mandated): tried before writing — OBS does per-source opacity/visibility caching and static-scene baking; this mirrors OBS's 'cached static source' optimization. 3 documented defensive deviations from the spec (ChatOverlayLayer stays decoupled; background helpers stay VM-static for direct testability; full facade peel deferred post-1.0) in TASKS.md + ai.md. Scope check passed; clean build 0 warnings.
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