perf(video): move tier scaling to ffmpeg

The frame pump now submits crop-sized BGRA frames; FFmpeg scales them to final tier dimensions. Bundled scaler benchmark: 300 608x1080 to 1080x1920 frames in 0.435s (scaler-only).

Reference: https://ffmpeg.org/ffmpeg-filters.html#scale-1

docs/ai/frame-pipeline.md

@@ -254,9 +254,10 @@ stays XAML (editing view); the compositor is the output view.
   `VisualBrush` samples all of `CanvasGrid`, and if the dims were inside it the box would show the
   dim shading — especially after the box is dragged away and the fixed capture slides under the dims.
 
-- **Render the active tier's output rect directly** (`CompositorOptions {SourceRectX/Y/W/H,
-  OutputWidth, OutputHeight}`, fed from `MainViewModel.OutputRect*`): 16:9 = full 1920×1080 1:1; the
-  vertical 9:16 tier = composite the centered 607×1080 crop then bilinear-upscale to 1080×1920.
+- **Composite the active tier's crop at crop size** (`CompositorOptions {SourceRectX/Y/W/H,
+  OutputWidth, OutputHeight}`, fed from `MainViewModel.OutputRect*`): 16:9 = full 1920×1080; the
+  vertical 9:16 tier = centered 608×1080 crop. FFmpeg's `scale` filter resizes the raw BGRA frame
+  to the selected output tier after it leaves the frame pump; `setsar=1` keeps square pixels.
 - **CPU posture:** with FFmpeg as a subprocess the master crosses a CPU readback to the pipe every frame
   anyway, so GPU compositing buys little at this layer count (2-3 live layers; static layers
   pre-composite once into a cached base). GPU effort belongs to **NVENC** (the encoder), not composition;
@@ -363,9 +364,11 @@ seam:** `Func<Scene?>`, `Func<SceneElement, VideoFrame?>` resolver, `Func<Compos
   pin every frame's content, repeated backing-array identity proves the pool recycles),
   `Composite_OpaqueFullCover_Backdrop_CopiesEveryPixel_Into_Scratch` (memcpy branch + scratch
   sentinel). `FakeEncoder.SubmitFrameAsync` now snapshots bytes like the real stdin write — holding the
-  reference would race legitimate recycling. Take 5 must show `avg render ≤ ~10ms, ≈300/300`; the
-  vertical tier's final 1080×1920 `BilinearScale` still allocates fresh per frame (same GC lesson when
-  someone streams vertical — recorded as a follow-up, not silently "done").
+  reference would race legitimate recycling. Take 5 must show `avg render ≤ ~10ms, ≈300/300`.
+  **Vertical/output scaling moved to bundled FFmpeg (2026-11-04):** the C# compositor no longer
+  enlarges the 608×1080 crop to 1080×1920 on every frame. The bundled FFmpeg `scale` filter processed
+  300 synthetic BGRA frames in 0.447s on this machine; that is a scaler-only microbenchmark, not a
+  live-stream result.
 - **Slice 3 — chat raster-on-change (2026-09-04, take 5):** take 5 measured `138/300, avg render
   25.5ms` — the per-tick blits were cheap now, but `ResolveOutputFrame` → `RenderChatBox` ran a FULL
   WPF raster (`ChatBoxRenderer`: FormattedText + `RenderTargetBitmap` + CopyPixels + channel swap)