perf(compositor): slice 5 — paste cache for non-opaque layers (take-7/8 data)

The stamped build settled what slices 3-4 could not: chat cache works (resolve
~0.0ms) but render stayed 26-27ms -> 124-135/300. The cost was the compositor
re-rasterizing EVERY layer every tick: this Live scene re-samples chat (159k) +
web widget (271k) + image (95k) + cam (156k) ~ 680k px @ ~38ns — for layers
whose pixels do not change between chat/web/cam updates.

OBS shape: cache the surface, paste per tick. BlitCachedLayer rasterizes a
non-opaque layer ONCE into an element-space, transparent-based frame keyed by
(source-array identity, src W/H, ceil'd dst rect, round, mirror), then pastes:
integer position, row alpha-blend, opacity applied at paste. Producers hand out
fresh immutable arrays -> array-identity keys cannot serve stale content; dict
bounded (48, clears whole). Drag/opacity live in paste params, not keys, so
editing stops triggering resamples too. Opaque backdrop keeps the memcpy path;
the webcam keeps the direct path via its IsOpaque flag (revisit if take 9 is
borderline).

ONE integration test: PasteCache_RepeatRender_IsByteIdentical_And_ContentChange-
Propagates (byte-exact raster-vs-paste incl. round-clip margins, new-array
propagation); existing pixel suite guards sampler semantics. 85/85 across
compositor/pump/chat/capture/session classes, clean build 0 warnings. Also:
BuildStampTests.cs was written last commit but never staged — its own scope-check
slip, added here (the run had used the on-disk file; tracked now).

Docs same commit: ai.md slice 5 + stale 'general path only 130k' claim corrected,
TASKS.md take-9 gate, MyMistakes recipe (prove the stage; a fix that doesn't move
the stat wasn't the bottleneck), HANDOFF. take 9 expectation: 300/300, render
<= ~8ms -> saga closes, Unit B starts.

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 1–5 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 ran: render 58.9→25.5ms (`138/300` ≈ 2.2x still) — cause: the per-tick chat raster (`RenderFrame` hit `RenderTargetBitmap` every tick whenever the message buffer was non-empty — the buffer survives sessions); slice 3 shipped 2026-09-04: `ChatOverlayLayer` rasters on message/config change and blits a cached frame every tick (OBS text-source pattern; test `ChatOverlayLayerCacheTests`).** **take 6 ran WITHOUT attribution (35-41ms — build provenance unknown; slice 3 effectiveness UNCONFIRMED) → build-stamp shipped instead of guessing again: `Helpers/BuildStamp` GUID per build (csproj GenerateBuildStamp; incremental builds can no longer lie), wordmark superscript + startup.log line; stats split `render (resolve)` so the next take names the stage. **take 7 pending**: read stamp → confirm slice 3 build → stats must show `avg render ≤ ~10ms (resolve ≪), ≈300/300`. Known remaining churn (follow-ups, not silently done): vertical tier's final `BilinearScale` still allocates per frame; one slow tick (~15-25ms) per arriving chat message — debounced off-tick re-render if take 6 shows burst loss. **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
+### Status: ✅ Shipped `a9eb360` (2026-08-29) — code done (incl. manual-rename modal), build 0 warnings, 244/246 tests; **running-app verification (takes 1–5 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 ran: render 58.9→25.5ms (`138/300` ≈ 2.2x still) — cause: the per-tick chat raster (`RenderFrame` hit `RenderTargetBitmap` every tick whenever the message buffer was non-empty — the buffer survives sessions); slice 3 shipped 2026-09-04: `ChatOverlayLayer` rasters on message/config change and blits a cached frame every tick (OBS text-source pattern; test `ChatOverlayLayerCacheTests`).** **take 6 ran WITHOUT attribution (35-41ms — build provenance unknown; slice 3 effectiveness UNCONFIRMED) → build-stamp shipped instead of guessing again: `Helpers/BuildStamp` GUID per build (csproj GenerateBuildStamp; incremental builds can no longer lie), wordmark superscript + startup.log line; stats split `render (resolve)` so the next take names the stage. **takes 7–8 (stamped f190587b): chat fix CONFIRMED (`resolve ≈0`) but render stayed 26-27ms — compositor re-rasterizing STATIC layers every tick; slice 5 shipped: `BlitCachedLayer` pastes once-rasterized element-space layers (OBS surface-cache pattern; test `PasteCache_RepeatRender_IsByteIdentical_And_ContentChangePropagates`). **take 9 pending**: read stamp → stats must show `≈300/300, avg render ≤ ~8ms` → saga closes.** Known remaining churn (follow-ups, not silently done): vertical tier's final `BilinearScale` still allocates per frame; one slow tick (~15-25ms) per arriving chat message — debounced off-tick re-render if take 6 shows burst loss. **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`