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, 12 insertions, 2 deletions

diff --git a/src/effects/ntsc_effect.h b/src/effects/ntsc_effect.h
index d8624ea..4737291 100644
--- a/src/effects/ntsc_effect.h
+++ b/src/effects/ntsc_effect.h
@@ -1,12 +1,22 @@
-// NTSC post-process effect with fisheye distortion
+// NTSC post-process effects: RGB and YIQ input variants.
 #pragma once
 #include "effects/shaders.h"
 #include "gpu/wgsl_effect.h"
 
+// RGB input: converts RGB → YIQ internally (standard post-process use).
 struct Ntsc : public WgslEffect {
   Ntsc(const GpuContext& ctx, const std::vector<std::string>& inputs,
        const std::vector<std::string>& outputs, float start_time,
        float end_time)
       : WgslEffect(ctx, inputs, outputs, start_time, end_time,
-                   ntsc_shader_wgsl) {}
+                   ntsc_rgb_shader_wgsl) {}
+};
+
+// YIQ input: input texture already stores luma/chroma/phase (e.g. RotatingCube output).
+struct NtscYiq : public WgslEffect {
+  NtscYiq(const GpuContext& ctx, const std::vector<std::string>& inputs,
+          const std::vector<std::string>& outputs, float start_time,
+          float end_time)
+      : WgslEffect(ctx, inputs, outputs, start_time, end_time,
+                   ntsc_yiq_shader_wgsl) {}
 };