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.

Models/SceneElement.cs

@@ -19,6 +19,17 @@ public enum BorderAnimation
     Sparkles
 }
 
+/// <summary>Whether an element's pixel content is fixed (bake once into the
+/// compositor's cached base) or changes every frame (composite each tick).
+/// See ai.md "SceneGraph component + baked-crust compositor" (TASK 31).</summary>
+public enum ElementKind
+{
+    /// <summary>Pixels never change at runtime (images, background art).</summary>
+    Static,
+    /// <summary>Pixels change every frame (webcam, live capture, chat, web source).</summary>
+    Dynamic
+}
+
 /// <summary>
 /// A thing rendered in a scene: a multi-instance Source (image/background/text)
 /// or a singleton resource's per-scene config (webcam). Carries the shared
@@ -44,6 +55,11 @@ public abstract class SceneElement : INotifyPropertyChanged
     private string _name = string.Empty;
     public string Name { get => _name; set => Set(ref _name, value); }
 
+    /// <summary>Whether this element's pixels are fixed (Static) or change every
+    /// frame (Dynamic). The compositor bakes all Static layers below the first
+    /// Dynamic layer once (see <c>SceneGraph.GetSplitPoint</c>).</summary>
+    public virtual ElementKind Kind => ElementKind.Static;
+
     /// <summary>True only for singleton resources (webcam). Gates webcam-only UI.</summary>
     public virtual bool IsWebcam => false;
 

Models/Source.cs

@@ -63,6 +63,11 @@ public class Source : SceneElement
 
     public override bool IsDraggable => Type is SourceType.Image or SourceType.WebSource or SourceType.ChatBox;
 
+    /// <summary>Live captures, chat, web sources and media are dynamic; static art is fixed.</summary>
+    public override ElementKind Kind => Type is SourceType.DisplayCapture or SourceType.WindowCapture
+        or SourceType.ChatBox or SourceType.WebSource or SourceType.MediaSource
+        ? ElementKind.Dynamic : ElementKind.Static;
+
     public override bool IsWebSource => Type == SourceType.WebSource;
 
     /// <summary>The permanent scene background (live desktop/game capture). Never removable/reorderable.</summary>

Models/WebcamSceneConfig.cs

@@ -13,5 +13,6 @@ public class WebcamSceneConfig : SceneElement
     public string WebcamId { get; init; } = string.Empty;
     public override bool IsWebcam => true;
     public override bool IsDraggable => true;
+    public override ElementKind Kind => ElementKind.Dynamic;
     public override ImageSource? DisplaySource => VideoImageSource;
 }