fix(pump+compositor): take-3 starvation — deadline pacing + row-blit fast paths

Two defects made the producer 17x slow (37s record -> 2.1s/127-frame file,
rawvideo stamps by arrival): FramePump slept the FULL interval after each
render (period = render+submit+interval) and SceneCompositor did per-pixel
float sampling + Math.Round blends over all 2.07M master pixels, scanning the
whole destination per overlay (258ms avg render vs 1.5ms submit).

Both solutions are established, not invented — researched before coding per
the derivative-work rule:
- deadline pacing: OBS libobs/media-io/video-io.c video_thread (nextTick +=
  intervalTicks, sleep only the remainder, rebase on overrun, never burst)
- row blits: libyuv pattern (BSD-3, chromium.googlesource.com/libyuv/libyuv)
  — 1:1 aligned identity fast path, per-pixel alpha branch, integer
  fixed-point blend, overlay clipped to the intersection rect, skip the dead
  black pre-fill when the backdrop covers

ONE integration test: Pump_Paces_To_The_Deadline_Compensating_Render_Cost
(lands after a fake-seam lesson: pacing fakes must await, not complete
synchronously, or the pump loop runs inline on StartAsync and hangs vstest).
Clean build 0 warnings; FramePumpTests 10/10, SceneCompositor/SceneGraph/
SocialBar/StretchMath 20/20. Docs same commit: ai.md pipeline section,
TASKS.md TASK 18 (webcam-in-output + rename modal verified from take 3),
MyMistakes recipe, HANDOFF rewritten. Take 4 pending on the user's machine.

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 IN PROGRESS (2026-09-01 takes 1–2)**: files now actually land (dead ffmpeg pin re-pinned month-end, locator wrap), webcam-in-output + top-bar reachability + audio-loop NREs fixed en route; take 3 pending with pump stage-timing to resolve the ~2fps producer starvation (time-lapse/short-file/garbled-audio = one root cause, `FramePump stats` will name the stage)
+### 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)
 
 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`