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/output.md

@@ -67,7 +67,8 @@ a machine prerequisite (a manifest framework-dependency + graceful-failure path,
 ### Live encoder + RTMP push (TASK 4 ship step 3 — shipped 2026-08-12, plan in TASKS.md)
 
 The encoder is a **thin orchestrator over `ffmpeg.exe`** — no H.264/AAC code in the app. It spawns the
-subprocess (path from `IFfmpegLocator`), feeds raw BGRA master frames into stdin, and parses `-stats`
+subprocess (path from `IFfmpegLocator`), feeds raw BGRA compositor-crop frames into stdin, lets FFmpeg
+scale them to the selected output tier, and parses `-stats`
 stderr lines into `StreamHealth` (bitrate/FPS/duration, dropped-from-frame-count). `FfmpegEncoder`
 (`IFfmpegEncoder` seam) holds: `StartAsync` (locate → probe `-encoders` → spawn → stderr loop →
 drain loop), `SubmitFrameAsync` (slice 10: bounded-queue ENQUEUE — never a pipe write; the drain task
@@ -80,8 +81,10 @@ integration test fakes the whole subprocess (probe + encoder) with a Channel-bac
 `Complete()` is EOF (`null`), never a `ChannelClosedException`.
 
 **Decisions (locked):** args are pure (`FfmpegArgs.Build`, no string building in the encoder):
-`-f rawvideo -pix_fmt bgra -video_size WxH -framerate FPS -i pipe:0` (**NO `-re`** — the FramePump
-is the pacer since slice 9, 2026-09-10; `-re` added a second, fighting clock on the rawvideo demux)
+`-f rawvideo -pix_fmt bgra -video_size inputWxH -framerate FPS -i pipe:0` (**NO `-re`** — the FramePump
+is the pacer since slice 9, 2026-09-10; `-re` added a second, fighting clock on the rawvideo demux).
+When input and output dimensions differ, each output block adds `-vf scale=outputW:outputH:flags=bilinear,setsar=1`;
+otherwise frames pass through at their existing dimensions.
 + a **real audio input — the
 mixer writes IEEE-float stereo to a Windows named pipe** (`-f f32le -ar 48000 -ac 2
 -i \\.\pipe\ytllive_audio`, name via `EncoderOptions.AudioPipeName`; replaced the old `-f lavfi -i
@@ -99,7 +102,8 @@ other — while the dual-block machinery stays generic.**
 `FfmpegEncoder.StartAsync` throws unless at least one output is enabled.
 **Encoder choice is probed from the binary's `-encoders` listing** (`FfmpegEncoderPicker`, pure):
 NVENC → QSV → AMF → OpenH264 fallback, **never libx264** (GPL; see Licensing). `EncoderOptions.VideoEncoder`
-forces one and skips the probe. `EncoderOptions` also carries W×H/FPS/bitrate from the quality tier and
+forces one and skips the probe. `EncoderOptions` carries raw input W×H from the compositor crop, final
+output W×H/FPS/bitrate from the quality tier, and
 `GopSize` = FPS×4. Constructed by `MainViewModel` since ship step 5 (`new FfmpegEncoder(new FfmpegLocator())`),
 driven by the `FramePump` below.