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).

TASKS.md

@@ -973,7 +973,7 @@ click (volume sliders keep their manual `SetSliderValueFromClick`, harmless dupl
 
 **Goal:** record the stream output to a local file, with or without simultaneously streaming.
 
-### Status: ✅ Shipped `a9eb360` (2026-08-29) — code done (incl. manual-rename modal), build 0 warnings, 244/246 tests; **running-app verification (takes 1–3 done)**: files land (ffmpeg re-pinned month-end), **webcam-in-output + social bar visually CONFIRMED from take 3's extracted frame**, rename modal used for real (take 3 was named via it); take 3 exposed the ~2fps producer starvation and the pump stage-timing (`97ffc42`) named it in one line — `avg render 258.1ms` — **fixed 2026-09-03** (deadline pacing per OBS video-io.c + libyuv-style row blits in `SceneCompositor`, test `Pump_Paces_To_The_Deadline_Compensating_Render_Cost`); **take 4 pending** to verify 60fps lands (stats must show `n/300` and render ≤ ~16ms; next suspect if hiccuppy: 8.3MB/frame LOH alloc → buffer pool)
+### Status: ✅ Shipped `a9eb360` (2026-08-29) — code done (incl. manual-rename modal), build 0 warnings, 244/246 tests; **running-app verification (takes 1–4 done)**: files land (ffmpeg re-pinned month-end), **webcam-in-output + social bar visually CONFIRMED from take 3's extracted frame**, rename modal used for real (take 3 was named via it); take 3 exposed the ~2fps producer starvation and the pump stage-timing (`97ffc42`) named it in one line — `avg render 258.1ms` — **slice 1 fixed 2026-09-03** (deadline pacing per OBS video-io.c + libyuv-style row blits in `SceneCompositor`, test `Pump_Paces_To_The_Deadline_Compensating_Render_Cost`); **take 4: pacing held but render stayed 58.9ms** (2M-iteration row walk + 8.3MB/tick LOH) — **slice 2 shipped 2026-09-04**: `VideoFrame.IsOpaque` producer-contract flag → full-cover backdrop is one `Buffer.BlockCopy`; integer fixed-point bilinear general path; pump scratch pool (release strictly post-submit, owned-by-reference so cache frames are untouchable); dead per-tick `fromScene` render + the `fromSceneProvider` seam removed (BlendFrame uses `TransitionService.FromFrame` — the old render fed nothing). Tests `Pump_Pools_ScratchBuffers_Across_Frames_Without_Stale_Pixels` (the ONE) + `Composite_OpaqueFullCover_Backdrop_CopiesEveryPixel_Into_Scratch`; 59/59 per-class green, clean build 0 warnings. **take 5 pending**: stats must show `avg render ≤ ~10ms, ≈300/300`. Known remaining churn (follow-up, not silently done): the vertical tier's final `BilinearScale` still allocates per frame. **User UX spec (2026-09-04) queued behind this**: two-line top bar (LIVE rename, radio pills, Login/Logout button + avatar right-click Change Account, no account light; line 2 centered Start↔Stop grayed-until-armed) + up-front SaveFileDialog for REC (native overwrite prompt; retires the stop-time rename modal) + Go-Live dialog KEPT as preflight confirmation prefilled from the Text drawer — decisions captured in HANDOFF
 
 1. ✅ `EncoderOptions` extended with `StreamEnabled` / `RecordEnabled` / `RecordPath` (independent intent flags)
 2. ✅ `FfmpegArgs.Build` reworked into per-output blocks (stream `-f flv`, record `-f mp4`) via `AddVideoTags`