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/MainViewModel.Streaming.Operations.cs

@@ -56,7 +56,6 @@ public partial class MainViewModel : ViewModelBase
         ResetHealth(StreamStatus.Offline);
         _audioMixer.StartLive(EncoderOptions.DefaultAudioPipeName);
         IsRecording = true;
-        _webView2Manager?.SetCaptureInterval(33); // web layer ~30Hz while recording
         await _framePump.StartAsync();
     }
 
@@ -176,7 +175,6 @@ public partial class MainViewModel : ViewModelBase
             return;
         }
 
-        _webView2Manager?.SetCaptureInterval(33); // web layer ~30Hz while live
         await _framePump.StartAsync(); // never throws; failures log + surface via Failed
 
         // Health polling (TASK 5 item 3): poll immediately, then every 30s while
@@ -253,7 +251,6 @@ public partial class MainViewModel : ViewModelBase
         // monitoring); only the live/report pipe, the pump, and the session stop here.
         _audioMixer.StopLive();
         await _framePump.StopAsync();
-        _webView2Manager?.SetCaptureInterval(200); // idle pace — preview-only doesn't need 30Hz
 
         // Proper close-out (TASK 9 decision 6, built 2026-09-01): transition(complete)
         // AFTER the RTMP push is closed so no frames post-date the end — YouTube then