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, 20 insertions, 0 deletions

diff --git a/src/effects/ntsc_rgb.wgsl b/src/effects/ntsc_rgb.wgsl
new file mode 100644
index 0000000..09adbf1
--- /dev/null
+++ b/src/effects/ntsc_rgb.wgsl
@@ -0,0 +1,20 @@
+// NTSC post-process effect (RGB input): fisheye distortion, scanlines, color bleeding.
+// Input texture is in RGB color space; converted to YIQ for NTSC processing.
+#include "sequence_uniforms"
+#include "render/fullscreen_uv_vs"
+#include "math/noise"
+#include "math/color"
+#include "math/color_c64"
+#include "debug/debug_print"
+
+@group(0) @binding(0) var input_sampler: sampler;
+@group(0) @binding(1) var input_texture: texture_2d<f32>;
+@group(0) @binding(2) var<uniform> uniforms: UniformsSequenceParams;
+
+// RGB input: sample texture and convert to luma/chroma/phase
+fn sample_ntsc_signal(uv: vec2f) -> vec4f {
+  let rgba = textureSample(input_texture, input_sampler, uv);
+  return rgba_to_luma_chroma_phase(rgba, uv.y, YSIZE);
+}
+
+#include "render/ntsc_common"