fix(green_screen): make chromakey compositing portable across FFmpeg builds

The CI Linux FFmpeg build carried the keyed frame forward without an alpha
plane, so overlay drew opaque green over the background (corner stayed green)
instead of compositing — the E2E test failed there even though it passed on
macOS/Windows.

Force `format=yuva420p` immediately after chromakey so the keyed transparency
always has an explicit alpha plane, and size the background to the frame up
front (color=...:size=WxH, passing the probed width/height into
_process_chromakey) instead of scaling a 1x1 source with scale2ref — dropping
scale2ref also removes the format negotiation that discarded the alpha on some
builds. Output is flattened to yuv420p after the overlay.

tests/tools/test_green_screen_chromakey.py

@@ -50,16 +50,20 @@ def test_chromakey_filter_scales_background_to_frame():
             fd.mkdir()
             pd.mkdir()
             (fd / "frame_0000.png").write_bytes(b"stub")
-            tool._process_chromakey(fd, pd, "#0E172A", 1)
+            tool._process_chromakey(fd, pd, "#0E172A", 1, 320, 240)
     finally:
         GreenScreenProcessor.run_command = orig
 
     ffmpeg_cmds = [c for c in captured if c and c[0] == "ffmpeg"]
     assert ffmpeg_cmds, "no ffmpeg command built"
-    fc_idx = ffmpeg_cmds[0].index("-filter_complex")
-    fc = ffmpeg_cmds[0][fc_idx + 1]
-    assert "scale2ref" in fc, f"background not scaled to frame: {fc}"
+    cmd = ffmpeg_cmds[0]
+    fc = cmd[cmd.index("-filter_complex") + 1]
+    # Background must be sized to the frame, not the old 1x1 no-op.
+    assert "size=320x240" in " ".join(cmd), "background not sized to frame"
+    assert "size=1x1" not in " ".join(cmd), "still using the 1x1 background"
     assert "[0:v]scale=iw:ih[bg]" not in fc, "still using the 1x1 no-op scale"
+    # Alpha must be forced so keyed transparency survives on every FFmpeg build.
+    assert "format=yuva420p" in fc, f"keyed alpha not forced: {fc}"
 
 
 @pytest.mark.skipif(shutil.which("ffmpeg") is None or shutil.which("ffprobe") is None,
@@ -79,7 +83,9 @@ def test_chromakey_preserves_frame_size_and_keys(tmp_path):
         capture_output=True, check=True, timeout=60,
     )
 
-    ok = GreenScreenProcessor()._process_chromakey(frames_dir, processed_dir, "#0E172A", 1)
+    ok = GreenScreenProcessor()._process_chromakey(
+        frames_dir, processed_dir, "#0E172A", 1, 320, 240
+    )
     assert ok
 
     out = processed_dir / "frame_0000.png"