perf(capture): slice 8 — buffer ring + paste-cache Epoch; gen2 visibility (take-11 spikes)

Take 11 (c10ce06c) validated the off-UI architecture: typical frames land
work ~10ms + wait ~6.8ms = 16.7 exactly on the deadline; 212/300 best yet.
The ENTIRE remaining gap is periodic 35-65ms render spikes that WORSENED
across the take (189 -> 147) — the signature of gen2 GC pauses. Biggest
churner is structural: the screen capture minted a fresh ~8.3MB byte[] per
DWM frame (~500MB/s of LOH), a producer OBS never does (it owns fixed
surface pools).

- ScreenCaptureFrameSource: 4-deep buffer ring with size-matched slots (a
  <=17ms consumer cannot be lapped at 60Hz) + reused downscale row scratch.
- VideoFrame.Epoch: monotonic per producer frame. The paste cache keys on
  array IDENTITY, so recycled arrays MUST be distinguished — epoch joins the
  PasteKey. Producers handing fresh arrays leave it 0 (key unchanged effect).
- Stats print 'gen2 +N' per 5s window: next take acquits or convicts GC
  without another guess (rule: prove the stage).
- Test (the ONE): PasteCache_RecycledArrayWithNewEpoch_ReRasterizes_NotStaleHits
  — same array, new content, bumped epoch; fails on the old key by
  construction. 37/37 compositor/pump, clean build.
- Next suspect if gen2 stays hot: the 10Hz WebView2 capture (full-canvas PNG
  decode + fresh arrays on the UI thread) — recorded, untouched.

Creator audio ask queued in the same working session (+40% post-mix master
gain before the -1dBFS limiter) lands as its own commit next.

Services/VideoFrame.cs

@@ -13,6 +13,13 @@ public sealed class VideoFrame
     public byte[] BgraPixels { get; }
     public int Stride => Width * 4;
 
+    /// <summary>Producer frame epoch — a monotonically increasing number stamped by
+    /// sources that RECYCLE their pixel buffers (the screen-capture ring, take-11 spike
+    /// fix). Consumers that key caches on buffer identity (the compositor's paste cache)
+    /// MUST include it, or a recycled array false-hits with stale content. Producers
+    /// that hand out fresh arrays per frame leave it 0 — identity alone is then enough.</summary>
+    public long Epoch { get; init; }
+
     /// <summary>Producer contract: every alpha byte in <see cref="BgraPixels"/> is 255
     /// (DWM capture surfaces and MediaCapture video carry no alpha — the OS fills 255).
     /// Lets the compositor take a straight-copy fast path for a full-canvas opaque