perf(render): take-4 slice 2 — IsOpaque memcpy, integer bilinear, pump scratch pool

Take 4: pacing held (sync perfect) but avg render stayed 58.9ms — 2M
managed row-walk iterations + a fresh 8.3MB buffer every tick (LOH churn
into GC stalls inside the render measurement).

- VideoFrame.IsOpaque: producer-contract flag (screen capture + webcam —
  DWM/MF fill alpha 255; media/chat/web/static NOT flagged). Full-cover
  aligned opaque backdrop = ONE Buffer.BlockCopy; black pre-fill skipped
  when it covers.
- General BlitContent: integer 8.8 fixed-point bilinear + blend, row
  invariants hoisted, no per-pixel division/Math.Round. Within ±1 of the
  float reference (pixel tests allow ±2). Research per derivative-work
  rule: libyuv row/scale kernels (chromium.googlesource.com/libyuv/libyuv).
- FramePump scratch pool (max 4, length-keyed, owned-by-reference):
  release strictly AFTER SubmitFrameAsync returns (stdin write copies);
  Contains-guard makes the transition Cut alias safe.
- Removed the dead per-tick fromScene render + fromSceneProvider seam —
  BlendFrame consumes TransitionService.FromFrame captured at Start; the
  pump's render fed nothing. MainViewModel call site updated (signature).

Bugs caught by the pixel probes pre-ship (recorded MyMistakes): first
Bilinear double-shifted both stages (solid-255 sampled to ~1 -> general
path drew nothing); sentinel 0xAB collided with an x+y pixel. FakeEncoder
snapshots submitted frames (mirrors real copy semantics under recycling).

ONE integration test: Pump_Pools_ScratchBuffers_Across_Frames_Without_
Stale_Pixels (alternating backdrops + repeated backing identity). Direct
pin: Composite_OpaqueFullCover_Backdrop_CopiesEveryPixel_Into_Scratch.
Clean build 0 warnings; 59/59 per-class + RealApp boot-smoke. take 5
verdict: expect avg render <= ~10ms, ~300/300 frames. Docs same commit:
ai.md pipeline section, TASKS.md TASK 18, HANDOFF rewritten (Unit B spec
+ settled decisions queued).

Services/ScreenCaptureFrameSource.cs

@@ -177,12 +177,13 @@ public sealed class ScreenCaptureFrameSource : IScreenCaptureSource
             var scale = Math.Min(MaxBackgroundWidth / (double)sw, MaxBackgroundHeight / (double)sh);
             var dw = Math.Max(1, (int)(sw * scale));
             var dh = Math.Max(1, (int)(sh * scale));
-            return new VideoFrame(dw, dh, DownscaleBgra(data, sw, sh, srcStride, dw, dh));
+            // DWM delivers an opaque surface (alpha 255); bilinear keeps it 255.
+            return new VideoFrame(dw, dh, DownscaleBgra(data, sw, sh, srcStride, dw, dh)) { IsOpaque = true };
         }
 
         var pixels = new byte[count];
         Marshal.Copy(data, pixels, 0, pixels.Length);
-        return new VideoFrame(sw, sh, pixels);
+        return new VideoFrame(sw, sh, pixels) { IsOpaque = true };
     }
 
     // Bilinear downscale to the master frame. Reads each source row pair through