refactor: split LayoutStore by concern — 500-limit Phase 3, Commit E

LayoutStore.cs 1402 -> 47-line shell + 5 partials, grouped by functionality
(line count is a consequence, not the goal):

- LayoutStore.Migrations.cs: all DB schema migration (EnsureSchema, GetUserVersion,
  MigrateSourceTable/WebcamConfigTable/ToV3/SceneTable/SceneSocialBarColumn/
  SocialsTable/SocialEntryTable)
- LayoutStore.Load.cs: Load() scene/source read-back
- LayoutStore.Save.cs: Save() full scene tree write
- LayoutStore.Settings.cs: key/value settings + mic/record-folder/reusable-stream
  + license/transition/broadcast/hotkey/window-state GetSetting/UpsertSetting
- LayoutStore.Assets.cs: GetAssetBytes/UpsertAsset blob IO
- LayoutStore.cs: shell — Instance, fields, ctor, Dispose

Converted to public partial class LayoutStore : IDisposable. Pruned per-file
dead usings. Phase 3 DOWN: no production .cs exceeds 500 lines.

Zero behavior change; build 0 warnings; 246 pass, only the 2 known failures.
Docs: ViewModels/index.md Phase 3 tracker updated in same commit.

ViewModels/index.md

@@ -70,5 +70,6 @@ commit, tag `refactor-commit-N`, no per-commit push).
 - [x] **Commit B · `refactor-commit-B`** — **Streaming** → `MainViewModel.Streaming.cs` 789 → **330** + new **`MainViewModel.Streaming.Operations.cs`** (459 lines). Moved the go-live/record/health operation bodies (`StartSession`, `BeginRecordOnly`, `BeginGoLive`, `StartRecordFile`, `PrepareAndStartLiveAsync`, `PollHealthAsync`/`OnHealthPollTick`/`ApplyHealthIssue`, `StopStream`, `FinalizeRecordingAsync`, `UniquePath`, `ChooseRecordFolder`, `ResetRecordFolder`, `DefaultRecordFolder`, `OnFramePumpFailed`, `OnFramePumpHealthUpdated`, `ApplyHealth`, `ResetHealth`, `BuildEncoderOptions`) into the Operations partial; Streaming.cs keeps state + quality/resolution tiers + session timer + command declarations. Pruned dead usings both files. Pending: Background, SocialsDialogViewModel, LayoutStore.
 - [x] **Commit C · `refactor-commit-C`** — **Background** → `MainViewModel.Background.cs` 567 → **418** + new **`MainViewModel.Background.Model.cs`** (163 lines). Moved the static scene-model helpers (`LoadBackgroundImage`, `CreatePlaceholderSnapshot`, `EnsureBackground`, `CreateBackground`, `NormalizeBackgrounds`, `AllBackgrounds`, `ResolveAutoCaptureKey`, `MonitorKeyPrefix`) into the Model partial; Background.cs keeps capture/live state + operations (`UpdateActiveBackground`, `UpdateBackgroundImage`, `RefreshSnapshotsAsync`, `ReacquireScreenCaptures`, `OnScreenPreviewBitmapChanged`, `OnFullscreenMonitorChanged`, `RefreshBackgroundAutoCapture`, `BrowseBackground`, `ClearBackgroundCaptureAsync`, `RedesignateBackgroundAsync`, `ChangeBackgroundCaptureAsync`, `SetBackgroundCapture`, `DefaultBackgroundAsset`, `HealBackgrounds`, `StampDefaultBackgroundAsset`). Pending: SocialsDialogViewModel, LayoutStore.
 - [x] **Commit D · `refactor-commit-D`** — **SocialsDialog** → `SocialsDialogViewModel.cs` 515 → **334** + new **`SocialSlotViewModel.cs`** (183 lines). Moved the `DialogEntry` record + the row-level `SocialSlotViewModel` (six-fixed-slot row VM: service/logo/lock/edit/validate state + computed `Show*` flags) into its own file; `SocialsDialogViewModel.cs` keeps the dialog VM (sign-in gate, validation orchestration, commit). `DialogEntry` stays `public` in the same namespace so both files see it. Pending: LayoutStore.
+- [x] **Commit E · `refactor-commit-E`** — **LayoutStore** → `LayoutStore.cs` 1402 split into 6 concern-based partials: **`LayoutStore.cs`** (47: shell — `Instance`, fields, ctor + `EnsureSchema`, `GetUserVersion`, `Dispose`), **`LayoutStore.Migrations.cs`** (495: all `Migrate*` + `EnsureSchema`), **`LayoutStore.Load.cs`** (221: `Load`), **`LayoutStore.Save.cs`** (339: `Save`), **`LayoutStore.Settings.cs`** (292: mic/record-folder/reusable-stream + license/transition/broadcast/hotkey/window `GetSetting`/`UpsertSetting`), **`LayoutStore.Assets.cs`** (41: `GetAssetBytes`/`UpsertAsset`). **PHASE 3 COMPLETE — no production `.cs` over 500.** Pending: session wrap-up (HANDOFF.md) + push on approval.
 The partial `.cs` files land next to `MainViewModel.cs` in this folder as the split
 proceeds; each partial carries its own `using`s and re-declares nothing from core.