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

Services/Encoder/FramePump.cs

@@ -10,7 +10,7 @@ namespace ytLive.Services.Encoder;
 /// The live frame producer (TASK 4 ship step 5): the bridge between the capture
 /// managers + compositor and the encoder. While live it snapshots the active
 /// scene each tick, resolves every element to its latest frame, composites it
-/// into the tier's output frame, and paces frames into the encoder at the tier's
+/// into the tier's cropped input frame, and paces frames into the encoder at the tier's
 /// FPS. All collaborators are constructor-injected seams (scene, resolver,
 /// options, encoder factory, pacing delay) so the pump stays free of WPF and of
 /// the capture managers and is fully hermetic in tests.
@@ -722,11 +722,9 @@ public sealed class FramePump : IDisposable
         byte[]? scratch, Func<SceneElement, VideoFrame?> resolver,
         CompositeTimings? timings = null)
     {
-        // 1:1 guard: the cache stores the OUTPUT bytes and scratch is source-sized —
-        // at 1:1 they are the same buffer size. Off-size tiers (a vertical 1080×1920
-        // tier, an H264-1080p tier) fall back to a per-tick full render; the deployed
-        // config is master-sized and caching a per-tick fresh scale buffer is the known
-        // vertical-tier follow-up (see ai.md), not silently baked here.
+        // The cache stores the compositor frame at crop size; FFmpeg scales that frame
+        // to the final output dimensions. With product options, scratch and rendered
+        // frames are therefore the same size, including for vertical output.
         var useCache = scratch != null && options.SourceRectWidth == options.OutputWidth
             && options.SourceRectHeight == options.OutputHeight;
         var signature = useCache