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.

TASKS/task-17-web-source-rendering.md

@@ -11,7 +11,10 @@
 2. ✅ Schema v10: `WebUri TEXT` column on `Source` table + migration in `LayoutStore.cs`
 3. ✅ Persist `Source.WebUri` on save/load (currently in-memory only — lost on restart)
 4. ✅ Hidden off-screen `WebView2` control per web source — navigates to `WebUri`, renders in-app
-5. ✅ Frame capture from WebView2 (`CoreWebView2.CapturePreviewAsync`) → `WriteableBitmap` (BGRA8)
+5. ✅ Frame capture from WebView2 (**superseded 2026-09-14:** was `CoreWebView2.CapturePreviewAsync`
+   → `WriteableBitmap` (BGRA8); now composition-capture via
+   `CoreWebView2CompositionController` → `Windows.Graphics.Capture` — see the rendering-model
+   update below. PNG polling is deleted)
 6. ✅ Wire into `FramePump` resolver — `Source { Type: WebSource }` → latest WebView2 frame
 7. ✅ Wire into `SceneCompositor` — render web source as an image element at its position/size
 8. ✅ Preview shows live web content (not just a blank rectangle)
@@ -33,6 +36,18 @@ measure DOM stuff on NavigationCompleted (unsettled layout broke the image — `
 sizing the container to the crop (`ab29ec8` reverted — made it worse); a widget glow effect can
 fake a gap — verify with a plain widget first; measure rendered pixels instead.
 
+**Rendering model update (2026-09-14, COMPOSITION capture — the PNG poll is gone):** frame capture
+is no longer `CapturePreviewAsync` → PNG (35–165ms per full-HD encode+decode capped real cadence at
+~20Hz; item 5 below is superseded). Each session now renders through a
+`CoreWebView2CompositionController` visual into `Windows.Graphics.Capture`
+(`GraphicsCaptureItem.CreateFromVisual` of the root 1920×1080 `ContainerVisual`), pulled frame-driven
+from a free-threaded `Direct3D11CaptureFramePool` at the renderer's own rate (~60fps, parity with the
+browser). Straight-alpha readback, alpha-bbox crop + `CropBounds` per frame, epoch'd ring reuse, one
+shared crop-sized preview bitmap. The old `SetCaptureInterval` cadence hooks and `CaptureScheduler`
+are deleted; the hidden XAML `WebViewHostPanel` overlay and `CapturePreviewAsync` telemetry are gone.
+`TransparentBackgroundScript` is unchanged (a test pins it). Full design + the CoreMessaging DQ
+recipe in `../ai.md` → Slice 14. **Open: device verification of ~60fps web animation.**
+
 **Properties panel (2026-08-28):** web URI ✓/✕ icon buttons are `IsTabStop="False"` so Tab flows
 X→Y→W→H→URI; the ✕ button now clears the URI textbox (was reverting to the pre-accept snapshot).
 Slider style gained `IsMoveToPointEnabled="True"` — click-anywhere-on-bar jumps the thumb to the
@@ -42,6 +57,7 @@ click (volume sliders keep their manual `SetSliderValueFromClick`, harmless dupl
 
 - WebView2 is the only option for Windows — it's pre-installed on Windows 10 20H2+ and Windows 11
 - The web source is a standard element — positioned/sized/opacitied like any image source
-- Frame capture rate can be lower than video FPS (5-10 fps for web content is fine)
+- Frame capture is drive-by-renderer (composition capture, ~60fps parity with the browser since
+  2026-09-14; the old "5-10 fps is fine" poll model is deleted)
 - This enables Streamlabs/StreamElements overlays via web URLs