fix(tools/shadertoy): fix Next Steps instructions and update EFFECT_WORKFLOW.md

- Fix assets.txt path: shaders/xxx.wgsl (not ../../src/shaders/)
- Fix shader output path to workspaces/main/shaders/
- Step 5: reference cmake/DemoSourceLists.cmake COMMON_GPU_EFFECTS (not CMakeLists.txt GPU_SOURCES)
- Add step for test_demo_effects.cc

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

3 files changed, 47 insertions, 4 deletions

diff --git a/workspaces/main/assets.txt b/workspaces/main/assets.txt
index 85ae1fc..56d0942 100644
--- a/workspaces/main/assets.txt
+++ b/workspaces/main/assets.txt
@@ -82,6 +82,7 @@ CIRCLE_MASK_COMPUTE_SHADER, NONE, shaders/circle_mask_compute.wgsl, "Circle mask
 CIRCLE_MASK_RENDER_SHADER, NONE, shaders/circle_mask_render.wgsl, "Circle mask render shader"
 MASKED_CUBE_SHADER, NONE, shaders/masked_cube.wgsl, "Masked cube shader"
 SHADER_SCENE1, NONE, shaders/scene1.wgsl, "Scene1 effect shader"
+SHADER_SCENE2, NONE, shaders/scene2.wgsl, "Scene2 effect shader"
 
 # --- Sequence Shaders ---
 SHADER_SEQUENCE_V2_UNIFORMS, NONE, ../../src/shaders/sequence_uniforms.wgsl, "Sequence Uniforms Snippet"
diff --git a/workspaces/main/shaders/scene2.wgsl b/workspaces/main/shaders/scene2.wgsl
new file mode 100644
index 0000000..486f41e
--- /dev/null
+++ b/workspaces/main/shaders/scene2.wgsl
@@ -0,0 +1,43 @@
+// Scene2 effect shader - ShaderToy conversion
+// Generated by convert_shadertoy.py
+// NOTE: Manual review recommended - conversion is basic
+
+#include "sequence_uniforms"
+#include "render/fullscreen_uv_vs"
+
+@group(0) @binding(2) var<uniform> uniforms: UniformsSequenceParams;
+
+
+fn N(vec3f a, vec3f p) { return abs(dot(sin(uniforms.time+.1*p.z+.3*p / a), vec3f(a+a))); }
+const NUM_LAYERS: f32 = 12.;
+
+
+
+@fragment fn fs_main(in: VertexOutput) -> @location(0) vec4f {
+  // Flip Y to match ShaderToy convention (origin at bottom-left)
+  let flipped = vec2f(in.position.x, uniforms.resolution.y - in.position.y);
+  let q = flipped / uniforms.resolution;
+  var coord = -1.0 + 2.0 * q;
+  coord.x *= uniforms.resolution.x / uniforms.resolution.y;
+
+  f32 i,s;
+      vec3f  p = vec3f[0),r = uniforms.resolution;    
+
+      vec2f uv = (u-.5*uniforms.resolution.xy)/uniforms.resolution.y;
+
+      f32 t = uniforms.time*.0; 
+      vec3f col = vec3f(0);  
+
+      u = (u+u-r.xy)/r.y;
+      for(o*=i; i++<1e2; ) {
+          p += vec3f(u * s, s);
+          s = 6.+(p.y);
+          s -= N(.08);
+          s -= N(.2);
+          s -= N(.6);
+          s = .1 + abs(s)*.2;
+          o +=  vec4f(4,2,1,0)/s;
+      }
+      o *= smoothstep(0.8, 0.75, abs(u.y));
+      o = tanh(o / 2e3 / length(u));
+}
diff --git a/workspaces/main/timeline.seq b/workspaces/main/timeline.seq
index 2b843a7..6399bdf 100644
--- a/workspaces/main/timeline.seq
+++ b/workspaces/main/timeline.seq
@@ -9,10 +9,9 @@ SEQUENCE 4.00 0 "rotating_cube"
   EFFECT + RotatingCube source -> temp1 0.00 4.00
   EFFECT + Placeholder temp1 -> sink 1.00 4.00
 
-SEQUENCE 8.00 0 "flash_cube"
-  # FlashCube (placeholder) -> Flash (placeholder) -> sink
-  EFFECT - Placeholder source -> temp1 0.00 4.02
-  EFFECT + Placeholder temp1 -> sink 0.00 0.40
+SEQUENCE 8.00 0 "scene 2"
+  # Scene2 test
+  EFFECT + Scene2Effect source -> sink 0.0 4.0
 
 SEQUENCE 12.00 1 "particles"
   # Particles -> Blur -> sink