handoff: Phase 3 complete (all production .cs <= 500), push pending for A-E + docs

Rewrite HANDOFF.md: Phase 2 is pushed at 1f4624c (stale claim corrected);
Phase 3 commit rows A-E + Controls/index.md listed as local-only, push pending
on explicit approval. Records the re-structuring principle (split by concern,
500-line count is a ceiling not the goal) and the session's truncation-accident
lesson. MyMistakes.md gains the never-awk-into-the-source-you-read rule.

HANDOFF.md

@@ -2,45 +2,50 @@
 
 ## Branch / Commit State
 
-**`main`**, working tree is **CLEAN**.
+**`main`**, working tree is **CLEAN**. `origin/main` == `1f4624c` (pushed).
 
-Both split phases are now complete. Phase 1 (view-model partials) is pushed;
-Phase 2 (MainWindow.xaml → UserControls) is complete but **local-only**.
+**Pushed** (reached `origin/main` == `1f4624c`, incl. tags): Phase 1 commit `0`
++ refactor commits `1..11`, **and Phase 2 commits `12..18`** (they were pushed
+at the Phase-2 milestone; the old "Phase 2 local-only" claim below was written
+before that push and is stale).
 
-**Pushed** (reached `origin/main` == `14d3ed4`, incl. tags): Phase 1 commit `0`
-and refactor commits `1..11` (`refactor-commit-0..11`), plus policy/HANDOFF.
+**LOCAL ONLY (Phase 3 — A, B, C, D, E) + the Controls doc commit** — policy
+2026-08-30: no per-commit push; `git push` only at a user-agreed milestone /
+"push it". **Push pending on explicit user approval:**
 
-**LOCAL ONLY (Phase 2 — 12, 13, 14, 15, 16, 17, 18)** — policy 2026-08-30: no
-per-commit push; `git push` only at a user-agreed milestone / "push it".
-**Push pending on explicit user approval:**
-
-| Commit | Hash | Tag | Control | From Loc |
-|--------|------|-----|---------|----------|
-| 12 | `bc23b64` | `refactor-commit-12` | `Controls/BottomBar` | footer |
-| 13 | `6914252` | `refactor-commit-13` | `Controls/HealthBanner` | row 1 banner |
-| 14 | `90f5ad5` | `refactor-commit-14` | `Controls/TopBar` | row 0 top bar |
-| 15 | `c5e3466` | `refactor-commit-15` | `Controls/PreviewPane` | center preview |
-| 16 | `73397cf` | `refactor-commit-16` | `Controls/SceneThumbnailStrip` | row 0 strip |
-| 17 | `c9fd1bd` | `refactor-commit-17` | `Controls/LeftPanel` | row 1 col 0 |
-| 18 | `544208e` | `refactor-commit-18` | `Controls/OverlayHost` | overlay dialogs |
+| Commit | Hash | Tag | Payload |
+|--------|------|-----|---------|
+| docs | `cb54637` | — | `Controls/index.md` (missing per-directory map) |
+| A | `6944db8` | `refactor-commit-A` | split `MainViewModel.cs` core → **495** |
+| B | `31362d1` | `refactor-commit-B` | split `MainViewModel.Streaming.cs` → **330** |
+| C | `fab2e09` | `refactor-commit-C` | split `MainViewModel.Background.cs` → **418** |
+| D | `d3271a0` | `refactor-commit-D` | trim `SocialsDialogViewModel.cs` → **334** |
+| E | `dcb3637` | `refactor-commit-E` | split `LayoutStore.cs` → 6 partials, **≤495** |
 
 ## What's In Flight
 
-Nothing code-in-flight — working tree clean, **Phase 2 complete**.
+Nothing code-in-flight — working tree clean, **Phase 3 complete**.
 
-**Phase 2 result:** `MainWindow.xaml` carved 1,681 → **123 lines** (a shell over
-6 UserControls under `Controls/`, namespace `ytLive.Controls`). Every change:
-0-warning build, `scripts/verify.sh` (246 pass + 2 known only), explicit-file
-`scripts/scope-check.sh`, `ViewModels/index.md` in the SAME commit, tagged
-`refactor-commit-N`, no per-commit push.
+**Phase 3 result:** every production `.cs` is now **≤ 500 lines** (checked via
+`git ls-files '*.cs'` minus tests; nothing exceeds 500). The 500-line count is a
+**hard ceiling, not the goal** — each file was split **grouped by functionality**
+so an AI can process one concern per file. Splits were **concern-driven**:
+- `MainViewModel` → `.Sources.cs`/`.Web.cs`, `Streaming.cs` + `Streaming.Operations.cs`,
+  `Background.cs` + `Background.Model.cs`; `SocialSlotViewModel.cs` (row VM).
+- `LayoutStore` (was 1402) → `Migrations.cs`/`Load.cs`/`Save.cs`/`Settings.cs`/
+  `Assets.cs` + a 47-line shell; `public partial class LayoutStore : IDisposable`.
+
+Every change: 0-warning build, `scripts/verify.sh` (246 pass + 2 known only),
+explicit-file `scripts/scope-check.sh`, `ViewModels/index.md` tracker in the
+SAME commit, tagged `refactor-commit-N`, no per-commit push.
 
 **Next:** 
-- If user says "push": `git push origin main && git push --tags` (7 commits 12-18
-  + their tags).
-- Then optional Phase 3 if queued. Nothing is queued beyond Phase 2.
+- If user says "push": `git push origin main && git push --tags` (6 commits:
+  `cb54637` + A–E, and tags `refactor-commit-A..E`).
 
-**Pending bus:** `ViewModels/index.md` Phase 2 tracker has rows 12–18 (closed);
-Phase 1 tracker closed. `Controls/` has `index.md`-style facts only if needed.
+**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`
+(464), `ViewModels/MainViewModel.Audio.cs` (408).
 
 ## Landmines
 
@@ -69,31 +74,29 @@ Phase 1 tracker closed. `Controls/` has `index.md`-style facts only if needed.
 ## Todo state
 
 - MainViewModel refactor: **Phase 1 complete (11/11 partials)**, pushed.
-- MainWindow.xaml refactor: **Phase 2 complete (6/6 controls)**, local-only.
+- MainWindow.xaml refactor: **Phase 2 complete (6/6 controls)**, pushed at `1f4624c`.
+- 500-line compliance: **Phase 3 complete (all 5 offenders ≤ 500)**, local-only A–E.
 - 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.
 - TASK 22 (audio sync offset): not started — required for v1.
 
-## Session summary (2026-08-30)
+## Session summary (2026-08-31)
 
-- Ran the **MainWindow.xaml split to completion** under the standing "continue
+- Ran the **500-line Phase 3 split to completion** under the standing "continue
   without stopping" directive — no interruption prompts, no `question` tool.
-- Commits 12–18 landed locally, each: cut → 0-warning build → verify (246 pass +
-  2 known) → scope-check → index.md in-commit → `git tag refactor-commit-N`. No push.
-- **Commit 15 (PreviewPane** the hardest: 1920×1080 canvas + drag/select/resize/
-  hit-test, webcam menu handlers, social glow, audio meters. A dropped outer
-  `Border` (background/glow/margin) was caught and restored same-commit.
-- **Commit 17 (LeftPanel)** the biggest: property editors + chat autoscroll
-  (moved to self-subscribe on `Loaded`), drag-to-reorder, snapshot revert/accept
-  family; window routes via `_leftPanel.IsClickInside` and
-  `_leftPanel.OnSelectionChanged(vm)`.
-- **Commit 18 (OverlayHost)** the scope-decision: moved only the three dialogs;
-  kept `WebViewHostPanel` + `ToastArea` as window-root stubs because their
-  name-based lookups (WebView2 init, toast `InArea`) would break if buried in a
-  control — a spin-guard decision, not a guess.
-- Two XAML extraction gotchas recorded: (a) a moved block losing its `xmlns`
-  → MC2000 "key null" (Line n col m), fixed by adding `xmlns:models` on the
-  control root; (b) re-deriving line numbers after each edit before `sed`/awk
-  deletion ranges (the edit tool shifts numbers).
-- Push checkpoint reached; awaiting explicit "push" for commits 12–18 + tags.
+- Commits A–E landed locally, each: cut → 0-warning build → verify (246 pass + 2
+  known) → scope-check → index.md in-commit → `git tag refactor-commit-{A..E}`.
+- **Re-structuring principle (user correction, 2026-08-31):** split by
+  **functionality**, so an AI can process one concern per file; the 500-line
+  count is a **ceiling**, NOT the target. Do not pad or reshape code to a
+  number — group by concern and only split enough to stay under the ceiling.
+- **Commit E (LayoutStore)** the biggest: 1402 → 6 concern-based partials
+  (Migrations/Load/Save/Settings/Assets/shell), default partial split.
+- **Accident caught & recovered (Commit D):** a `> ViewModels/SocialsDialogViewModel.cs`
+  write that read from the file it was simultaneously truncating left the file at
+  1 line. Fixed IMMEDIATELY via `git checkout --` and re-ran the split. **Lesson**
+  (goes in `MyMistakes.md`): never `awk … > SRC` where `awk` also reads `SRC`;
+  stage every multi-block cut into temp files first, then assemble. All later
+  LayoutStore cuts did exactly that (read original once into temp files).
+- Push checkpoint reached; awaiting explicit "push" for `cb54637` + A–E + tags.