test(chroma): commit the real green-screen clip as a development fixture

The creator's test_greenscreen_moving_subject.mp4 (1920x1080, 5s, subject moving over a
green field) is what exposed the detector's brightness ceiling and proved the key colour must
come from the measured screen.

Development-only by construction, not by convention: ytLive.csproj lists Assets as explicit
<Resource Include> entries with no Assets/** glob, so a file that is not on that list is never
compiled in, copied to output, or published. The test project links this one path into its own
Fixtures output, so the repo holds ONE copy (23 KB).

ytLive.Tests/ytLive.Tests.csproj

@@ -23,6 +23,12 @@
 
   <ItemGroup>
     <None Include="Fixtures\**" CopyToOutputDirectory="PreserveNewest"/>
+    <!-- Real-clip fixtures live in the app's Assets folder so there is ONE copy in the repo. They are
+         NOT referenced by ytLive.csproj's explicit Resource list, so they are never bundled into a
+         build or a publish — development-only, exactly like the shapes we test them against. -->
+    <None Include="..\Assets\Videos\test_greenscreen_moving_subject.mp4"
+          Link="Fixtures\test_greenscreen_moving_subject.mp4"
+          CopyToOutputDirectory="PreserveNewest"/>
   </ItemGroup>
 
 </Project>