fix(web): web-layer capture cadence 10Hz → ~30Hz while recording — widget animations no longer play ~1/6 speed

The recording is 60fps but WebView2 capture was a blind 100ms DispatcherTimer = 10Hz;
each captured web frame repeated ~6x into the file caps web animation at the capture
rate, not the page's (user: 'the animation appears to be too slow').

- New CaptureScheduler (Services/CaptureScheduler.cs): per-session dispatcher timer
  that DROPS a tick while a capture is in flight (latest-wins, never queues) — the
  guard that makes a higher cadence safe: concurrent full-HD PNG CapturePreviewAsync
  calls (~10-30ms each, slow per WebView2Feedback#20) would stack CPU and publish
  stale-after-fresh. Effective cadence = max(interval, capture duration).
- Cadence: SetCaptureInterval(33) on record/stream start, (200) idle — applied via
  MainViewModel.Streaming.Operations.cs.
- De-throttle the hidden page: shared CoreWebView2Environment created BEFORE
  EnsureCoreWebView2Async with --disable-backgrounding-occluded-windows
  --disable-renderer-backgrounding --disable-features=CalculateNativeWinOcclusion.
  Off-screen WebView2 is a hidden page when the host window is unfocused/covered and
  Chromium then parks rAF and clamps timers to ~1s (WebView2Feedback#1172/#3070,
  Chrome-88 timer-throttling blog).
- Telemetry: first 30 captures per session log elapsed ms (PNG encode + decode) to
  startup.log — that decides whether ~30Hz stays or drops to ~20Hz; the FramePump
  drops frames (never time-lapses, slice 10) if UI-thread GC churn starves it.
- ONE test: CaptureScheduler_Drops_Ticks_While_Capture_InFlight_And_Resumes
  (deterministic TCS-driven, no WebView2 runtime). Suite 290/291 — sole failure the
  pre-existing compositor pixel test.
- Docs same-commit: ai.md slice 11, MyMistakes.md, HANDOFF.

References: https://github.com/MicrosoftEdge/WebView2Feedback/issues/1172
https://github.com/MicrosoftEdge/WebView2Feedback/issues/3070
https://github.com/MicrosoftEdge/WebView2Feedback/issues/20
https://developer.chrome.com/blog/timer-throttling-in-chrome-88

ViewModels/MainViewModel.Streaming.Operations.cs

@@ -56,6 +56,7 @@ 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();
     }
 
@@ -175,6 +176,7 @@ 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
@@ -251,6 +253,7 @@ 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