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.

Services/index.md

@@ -28,6 +28,8 @@ External-facing logic: YouTube API, persistence. See
 | `ScreenCaptureFrameSource.cs` | One `Direct3D11CaptureFramePool` (free-threaded, 2 buffers) + session per target; frames → `SoftwareBitmap.CreateCopyFromSurfaceAsync` (BGRA, alpha ignored) → `VideoFrame`, bytes read via `WindowsRuntimeMarshal.TryGetDataUnsafe` (CsWinRT-safe — the `IMemoryBufferByteAccess` ComImport cast fails on every frame and is gone). Surfaces > 1920×1080 downscaled bilinearly to the master; conversion failures logged ≤ once/5 s. DRM content = black frames (OS limit). `CreateForMonitor`/`CreateForWindow`/`CreateForPicker` |
 | `ScreenCaptureManager.cs` | Screen-capture ownership mirroring `CameraManager`: refcounted by target key, one shared `WriteableBitmap`, dispatcher-coalesced latest-frame copies; `PreviewBitmapChanged`/`CaptureFailed` events; `ReleaseAllAsync` used on re-designation |
 | `ScreenCaptureSourceFactory.cs` | `Resolve(key)` parses `monitor:<n>` / `window:<hwnd>` / `picker:<name>` into a source; `PickAsync()` shows the OS `GraphicsCapturePicker` and returns the `picker:` key (transient — a reload falls back to auto-detection) |
+| `WebView2Manager.cs` | **Web-source ownership (TASK 17, composition-capture since 2026-09-14):** per registered `Source { Type: WebSource }` it owns a `CoreWebView2CompositionController` + its visual tree (`RootVisualTarget` child under a 1920×1080 root `ContainerVisual` on one `Windows.UI.Composition.Compositor` created on the UI thread via the CoreMessaging `CreateDispatcherQueueController` P/Invoke), a wrapped `WebCaptureFrameSource`, and the shared crop-sized `WriteableBitmap` preview (dispatcher-coalesced copy at Render priority, `PreviewBitmapChanged` raised only on recreation). Nav + `TransparentBackgroundScript` injection unchanged. Public ctor `(Dispatcher)`; internal seam ctor `(Dispatcher, Func<string, IScreenCaptureSource>?)` for hermetic tests (no WinRT). Caller: `MainViewModel.InitWebView2()` (now invoked from `MainWindow_Loaded` — the parent HWND must exist). See `../ai.md` "Slice 14" |
+| `WebCaptureFrameSource.cs` | The `IScreenCaptureSource` for a web session: owns the `GraphicsCaptureItem` (`CreateFromVisual`) + a free-threaded `Direct3D11CaptureFramePool` (2 buffers) + session; readback via `SoftwareBitmap.CreateCopyFromSurfaceAsync` (BGRA, **`BitmapAlphaMode.Straight`** — the compositor blends straight alpha); per-frame `FindContentBounds` alpha-bbox (internal static, unit-tested) stamps `VideoFrame.CropBounds`; ring up to 2 fresh frames + Epoch reuse; `IsOpaque=false`. Mirror of `ScreenCaptureFrameSource` |
 | `Compositor/SceneCompositor.cs` | **The output compositor (TASK 4 ship step 1)**: renders a scene into the encoder's master `VideoFrame` (tightly-packed BGRA8), mirroring the XAML preview minus editing chrome — backdrop → background → elements (`UniformToFill` cover-crop, round clip, mirror, opacity, border) → branding flash → social bar (optional `socialBarFrame` + `socialBarTop` in master space, blitted last so the bar sits above the flash; the old `BlitFlash` generalized to `BlitOverlay` with source offsets). Pure and WPF-free: frames injected via a `Func<SceneElement, VideoFrame?>` resolver (webcam → DeviceId, image → AssetId, backdrop → CaptureKey); output sized by `CompositorOptions` (16:9 = full master 1:1; vertical 9:16 = 607×1080 crop → 1080×1920 bilinear). Preview stays XAML (editing view); this is the output view — see `ai.md` "Scene compositor" |
 | `Compositor/CompositorOptions.cs` | The active tier's output rect (source space over the 1920×1080 master, integer-aligned — `MainViewModel.OutputRectX` can be 656.5) + target W×H |
 | `Compositor/StretchMath.cs` | Pure pixel math: the WPF `UniformToFill` cover-crop, clamped bilinear sample/scale (unit-tested half of the compositor) |