ntsc: factor common code into snippet; add RGB and YIQ input variants

- Extract shared NTSC logic into render/ntsc_common.wgsl snippet
- sample_ntsc_signal() hook decouples input format from processing
- ntsc_rgb.wgsl: RGB input (converts via rgba_to_luma_chroma_phase)
- ntsc_yiq.wgsl: YIQ passthrough for RotatingCube output
- Add NtscYiq WgslEffect thin wrapper; register both in tests

handoff(Claude): NTSC refactor complete; NtscYiq ready for timeline use with RotatingCube.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/src/tests/gpu/test_demo_effects.cc b/src/tests/gpu/test_demo_effects.cc
index b9d1463..9d8cb7d 100644
--- a/src/tests/gpu/test_demo_effects.cc
+++ b/src/tests/gpu/test_demo_effects.cc
@@ -76,6 +76,9 @@ static void test_effects() {
       {"Ntsc", std::make_shared<Ntsc>(
                    fixture.ctx(), std::vector<std::string>{"source"},
                    std::vector<std::string>{"sink"}, 0.0f, 1000.0f)},
+      {"NtscYiq", std::make_shared<NtscYiq>(
+                      fixture.ctx(), std::vector<std::string>{"source"},
+                      std::vector<std::string>{"sink"}, 0.0f, 1000.0f)},
   };
 
   int passed = 0;