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.

HANDOFF.md

@@ -2,13 +2,15 @@
 
 ## Branch / Commit State
 
-**`main`**, working tree is **CLEAN**. `origin/main` == `85893ea` (pushed).
+**`main`**. TASK 31 (SceneGraph + baked-crust compositor) implemented, tests green,
+docs updated — **uncommitted**, ready to commit as one work unit (pending push at
+the user's checkpoint).
 
 **Pushed** (reached `origin/main` == `85893ea`, incl. tags): Phase 1 commit `0` +
 refactor commits `1..11` + **Phase 2 commits `12..18`** + the `Controls/index.md`
 docs commit `cb54637` + **Phase 3 splits A–E** (tags `refactor-commit-A..E`) +
 **Commit F (Recording, `refactor-commit-F`)** + **Commit G (ChatOverlayLayer,
-`refactor-commit-G`)**. Everything is pushed; nothing local-only.
+`refactor-commit-G`)**. Everything before TASK 31 is pushed; nothing local-only.
 
 **Commit log (all pushed):**
 
@@ -25,10 +27,16 @@ docs commit `cb54637` + **Phase 3 splits A–E** (tags `refactor-commit-A..E`) +
 
 ## What's In Flight
 
-Nothing code-in-flight — working tree clean.
-
-**TASK 31 designed:** SceneGraph component + baked-crust compositor optimization.
-Design captured in `TASKS.md`, pattern note in `ai.md`. Ready to implement.
+**TASK 31 (SceneGraph + baked-crust compositor) implemented — uncommitted, tests green.**
+Files touched (see `git status`): `Models/SceneElement.cs`, `Models/Source.cs`,
+`Models/WebcamSceneConfig.cs`, `Services/SceneGraph.cs` (new), `Services/Compositor/SceneCompositor.cs`,
+`Services/Encoder/FramePump.cs`, `ViewModels/MainViewModel.cs`, `MainViewModel.Scenes.cs`,
+`MainViewModel.Sources.cs`, `MainViewModel.Webcam.cs`, `MainViewModel.Background.cs`,
+`ytLive.Tests/SceneGraphTests.cs` (new), `ai.md`, `TASKS.md`, `HANDOFF.md`.
+Verification: SceneCompositorTests 4, StretchMathTests 4, BackgroundTests 16,
+SceneCatalogTests 18, LayoutStorePersistenceTests 12, FramePumpTests 9,
+SceneGraphTests 1 — all green. RealAppHost GUI tests hang headless (pre-existing).
+Next: run `scripts/verify.sh`/`scope-check.sh`, commit, await user's push go-ahead.
 
 **The directive (2026-08-31, user):** rewrite the project, breaking files into
 **functional components to compliment AI retrieval/processing** — NOT line-count
@@ -48,11 +56,13 @@ extracted services** (`AudioMixer`, `CameraManager`, `ScreenCaptureManager`,
 `ChatBoxRenderer`, `SocialValidator`). They have no cohesive owner-state blob to
 peel — forcing extraction adds coupling. **Chat was the one clean peel.**
 
-**Next (recommended):** the one remaining *genuine* big decomposition is the
-**scene-graph segment** (the `Scenes`/`StagedScene`/element-inventory backbone
-every feature hangs off). This is a real architectural redesign — a `SceneGraph`
-owner-object with narrow seams — that touches the binding surface and needs
-design review, NOT an unsupervised autonomous peel. Do not attempt it blind pre-1.0.
+**Next (recommended):** TASK 31 landed the *core* of the scene-graph architecture
+(the `SceneGraph` owner-object owning the collection + mutation surface, and the
+baked-crust compositor optimization) without the full binding-surface peel. The
+remaining step is the ViewModel-as-thin-facade migration (moving the many
+`Scenes`/`StagedScene`/`LiveScene` binding call sites onto the graph) — deferred
+as post-1.0 in `TASKS.md` because it is exactly the "unsupervised blind peel"
+regression risk. Do it at a real Windows session where the GUI tests can run.
 
 **Pending bus:** `ViewModels/index.md` Phase 3 tracker has rows A–E (closed);
 Phase 3 marked COMPLETE. Watch-list (under limit, no action): `Controls/PreviewPane.xaml.cs`
@@ -88,7 +98,7 @@ Phase 3 marked COMPLETE. Watch-list (under limit, no action): `Controls/PreviewP
 - MainWindow.xaml refactor: **Phase 2 complete (6/6 controls)**, pushed at `1f4624c`.
 - 500-line compliance: **Phase 3 complete (A–E, all ≤500)**, pushed.
 - **True decomposition: Commit G — `Services/ChatOverlayLayer.cs` shipped + pushed** (first owner-state component; pattern recorded in `ai.md`).
-- **Next (design-needed, do NOT blind-peel):** the scene-graph segment (`SceneGraph` owner-object) — the one remaining genuine big decomposition; touches binding surface, needs review.
+- **TASK 31 — SceneGraph + baked-crust compositor: implemented, tests green, uncommitted** (core optimization + collection/mutation ownership + integration test; see `TASKS.md` for the 3 documented defensive deviations). The full ViewModel-as-thin-facade peel (moving the entire binding surface) is intentionally deferred post-1.0 — flagged in `TASKS.md`.
 - TASK 3: 27/30 (preview compositor 16, text source 17, alerts 20 still open).
 - TASK 4: ✅ shipped. TASK 9: items 1–3 shipped; 4–7 open. TASK 10: steps 1–7; Velopack pending.
 - TASK 18: shipped, creator verification pending. TASK 19/23, 20, 21-A: shipped.
@@ -114,3 +124,10 @@ Phase 3 marked COMPLETE. Watch-list (under limit, no action): `Controls/PreviewP
   `git checkout --`, re-ran from temp files. **Lesson** in `MyMistakes.md`: never
   `awk … > SRC` while awking SRC; stage cuts into temp files, then assemble.
 - Everything through G is **pushed to `origin/main` (`85893ea`)** with tags A–G.
+- **TASK 31 implemented (uncommitted, tests green):** added `ElementKind` (Static/Dynamic);
+  created `Services/SceneGraph.cs` owning the collection + mutation surface + bake cache;
+  made `SceneCompositor` split-aware (`BakeStaticBase`/`CompositeLayers`/`Render(staticBase,split)`);
+  wired `FramePump` to the optimized path; invalidated the bake on static mutations; added
+  `SceneGraphTests.BakedStaticBase_WithDynamicLayer_CompositesCorrectly`. 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). See `TASKS.md` → TASK 31.