feat: web sources frame-captured via composition (CoreWebView2CompositionController → Windows.Graphics.Capture); PNG poll + CaptureScheduler deleted

The ~30Hz CapturePreviewAsync PNG poll capped real cadence at ~20Hz
(35–165ms full-HD encode+decode), so a 60fps widget still juddered at ~1/6
speed. Replaced polling with frame-driven capture of the composition
controller's root visual — the mechanism WebView2CompositionControl and
Flutter's webview_windows use (graphics_context.cc captures the root
surface_ visual via CreateGraphicsCaptureItemFromVisual; reference:
github.com/microsoft/Windows.UI.Composition.WinUI / flutter-internal
webview_windows). Frames now arrive at the renderer's own pace; capture
memory is epoch'd ring reuse + one crop-sized shared WriteableBitmap.

New Services/WebCaptureFrameSource.cs owns GraphicsCaptureItem + free-
threaded Direct3D11CaptureFramePool + session (Straight alpha readback,
per-frame FindContentBounds → CropBounds). WebView2Manager reworked around
per-session composition controllers + one UI-thread Compositor created via
the CoreMessaging CreateDispatcherQueueController P/Invoke (the 19041
projection lacks CreateOnCurrentThread); internal seam ctor
(Dispatcher, Func<string,IScreenCaptureSource>?) for hermetic tests.
CaptureScheduler.cs deleted; the three SetCaptureInterval cadence hooks
removed; InitWebView2() moved from MainWindow ctor to Loaded (a parent
HWND must exist for the composition controller); the hidden WebViewHostPanel
overlay deleted. TransparentBackgroundScript unchanged.

Tests: WebView2ManagerTests reworked — 4 control-size + scheduler tests
dropped, FindContentBounds tests moved to WebCaptureFrameSource, ONE
integration test (Frames_PublishCroppedPreview_And_CoalesceToLatest_CarryingCropBounds)
drives the seam with a FakeWebSource + background-STA DispatcherPump.
Suite 293/293, 0 warnings.

NOTE: composition path NOT yet verified on a device — the take is the
next step. Web work committed locally only (no push per standing rule).

Docs same-commit: ai.md Slice 14 + supersede marker on Slice 11, HANDOFF,
MyMistakes (CoreMessaging DQ + namespace-landmine recipe), TASK 17,
Controls/ViewModels/Services indexes.

ViewModels/index.md

@@ -60,7 +60,7 @@ Progress (Commit N/12 · tag):
 - [x] **Commit 15 · `refactor-commit-15`** — **PreviewPane** → `Controls/PreviewPane.xaml(.cs)` (Row 1 Col 1 center preview pane). Moved the 1920×1080 canvas (OverlayCanvas/SelectionOverlay/BrandFlashLayer/SocialBarElement), text pull-out, and the live-controls row (TRAX/game/mic meters + sliders + speakers + socials button). Code-behind moved: preview drag/select/resize/hit-test (Preview_Mouse*, HitHandle, HitElement), webcam-context-menu handlers (WebcamMenu_*), social-bar glow (SocialBar_*, SocialsButton_*, SocialGlowTick), audio sliders/speakers (VolumeSlider_*, GameVolumeSlider_*, MicSpeaker_*, GameSpeaker_*, SetSliderValueFromClick, FindParent), TraxButton_*. Window keeps the source-list left panel + thumbnail strip + edit UI, and routes selection-overlay via `_previewPane.UpdateSelectionOverlay(bool)` plus `_previewPane.IsClickInsidePreview/Drawer`. `BackgroundMenu_*` stayed in the window (used by the left-panel source context menu).
 - [x] **Commit 16 · `refactor-commit-16`** — **SceneThumbnailStrip** → `Controls/SceneThumbnailStrip.xaml(.cs)` (Row 0 scene-picker strip). `Thumbnail_MouseLeftButtonDown` moved into the control (transitions via `DataContext.TransitionToScene`).
 - [x] **Commit 17 · `refactor-commit-17`** — **LeftPanel** → `Controls/LeftPanel.xaml(.cs)` (Row 1 Col 0 left panel: layers/properties edit UI + live chat list). Moved ALL property-editor and chat handlers: `SourceNameBox_*`/`CommitSourceEdit`, `AddSourceButton_Click`, `OnChatMessagesChanged` (now self-subscribed on Loaded), `MirrorButton_Click`, `ShapeButton_Click`, `SourceMenu_Remove`, `BackgroundMenu_*`, `OpacitySlider_ValueChanged`, list drag-to-reorder (`_dragStartPoint`+`List_Preview*`+`EndListDrag`+`FindItemContainerAt`), and the snapshot family (`TakeSnapshot`, `WebcamBorder*`, `Countdown*`, `Web*`). Window exposes via `_leftPanel.IsClickInside(original)` (deselect guard) and `_leftPanel.OnSelectionChanged(vm)` (opacity label + snapshot). Pending: overlay host region.
-- [x] **Commit 18 · `refactor-commit-18`** — **OverlayHost** → `Controls/OverlayHost.xaml(.cs)` (settings/bug/feature/about dialogs). `BackToAbout_Click` moved with it (uses inherited `DataContext`). Window keeps the thin `WebViewHostPanel` + `ToastArea` root stubs so name lookups are untouched (WebView2 ctor init, `NotificationService.InArea("ToastArea")`) — toast/WebView2 behavior unchanged. **Phase 2 COMPLETE** — MainWindow.xaml carved down to a 123-line shell over 6 controls (TopBar, HealthBanner, BottomBar, SceneThumbnailStrip, LeftPanel, PreviewPane, OverlayHost). Pending: session wrap-up (HANDOFF.md) + push on approval.
+- [x] **Commit 18 · `refactor-commit-18`** — **OverlayHost** → `Controls/OverlayHost.xaml(.cs)` (settings/bug/feature/about dialogs). `BackToAbout_Click` moved with it (uses inherited `DataContext`). Window keeps the thin `WebViewHostPanel` + `ToastArea` root stubs so name lookups are untouched (WebView2 ctor init, `NotificationService.InArea("ToastArea")`) — toast/WebView2 behavior unchanged. **Phase 2 COMPLETE** — MainWindow.xaml carved down to a 123-line shell over 6 controls (TopBar, HealthBanner, BottomBar, SceneThumbnailStrip, LeftPanel, PreviewPane, OverlayHost). Pending: session wrap-up (HANDOFF.md) + push on approval. *(The `WebViewHostPanel` root stub is since GONE — the 2026-09-14 composition-capture redesign moved `InitWebView2()` to `MainWindow_Loaded` and deleted the panel; see `Controls/index.md`.)*
 
 **Phase 3 — 500-line compliance (production `.cs` only).** Bring all production
 source files under 500 lines. One roll-back commit per file; same drill