feat(sequence): port rotating_cube_effect to v2

- Add RotatingCubeEffectV2 with 3D rendering + depth buffer
- Create rotating_cube_v2.wgsl (hardcoded cube geometry)
- Simplified: no auxiliary mask texture dependency
- Declare depth node via NodeRegistry
- Update timeline_v2.seq rotating_cube sequence
- Add shader exports to shaders.{h,cc}
- All 36 tests passing

handoff(Claude): RotatingCube v2 complete, hybrid_3d next

3 files changed, 93 insertions, 4 deletions

diff --git a/workspaces/main/assets.txt b/workspaces/main/assets.txt
index fe15cdf..9d15213 100644
--- a/workspaces/main/assets.txt
+++ b/workspaces/main/assets.txt
@@ -35,6 +35,7 @@ SHADER_PARTICLE_COMPUTE, NONE, shaders/particle_compute.wgsl, "Particle Compute
 SHADER_PARTICLE_RENDER, NONE, shaders/particle_render.wgsl, "Particle Render Shader"
 SHADER_PARTICLE_COMPUTE_V2, NONE, shaders/particle_compute_v2.wgsl, "Particle Compute Shader V2"
 SHADER_PARTICLE_RENDER_V2, NONE, shaders/particle_render_v2.wgsl, "Particle Render Shader V2"
+SHADER_ROTATING_CUBE_V2, NONE, shaders/rotating_cube_v2.wgsl, "Rotating Cube Shader V2"
 SHADER_PASSTHROUGH, NONE, ../../common/shaders/passthrough.wgsl, "Passthrough Shader"
 SHADER_ELLIPSE, NONE, shaders/ellipse.wgsl, "Ellipse Shader"
 SHADER_PARTICLE_SPRAY_COMPUTE, NONE, shaders/particle_spray_compute.wgsl, "Particle Spray Compute"
diff --git a/workspaces/main/shaders/rotating_cube_v2.wgsl b/workspaces/main/shaders/rotating_cube_v2.wgsl
new file mode 100644
index 0000000..d7e4cae
--- /dev/null
+++ b/workspaces/main/shaders/rotating_cube_v2.wgsl
@@ -0,0 +1,89 @@
+// Rotating cube shader v2 (simplified, no masking)
+
+struct Uniforms {
+    view_proj: mat4x4<f32>,
+    inv_view_proj: mat4x4<f32>,
+    camera_pos_time: vec4<f32>,
+    params: vec4<f32>,
+    resolution: vec2<f32>,
+    aspect_ratio: f32,
+    _pad: f32,
+};
+
+struct ObjectData {
+    model: mat4x4<f32>,
+    inv_model: mat4x4<f32>,
+    color: vec4<f32>,
+    params: vec4<f32>,
+};
+
+@group(0) @binding(0) var<uniform> uniforms: Uniforms;
+@group(0) @binding(1) var<storage, read> object: ObjectData;
+
+struct VSOut {
+    @builtin(position) pos: vec4<f32>,
+    @location(0) world_pos: vec3<f32>,
+    @location(1) normal: vec3<f32>,
+};
+
+// Cube vertices (hardcoded)
+fn get_cube_vertex(vid: u32) -> vec3<f32> {
+    let positions = array<vec3<f32>, 36>(
+        // Front face
+        vec3<f32>(-1, -1,  1), vec3<f32>( 1, -1,  1), vec3<f32>( 1,  1,  1),
+        vec3<f32>(-1, -1,  1), vec3<f32>( 1,  1,  1), vec3<f32>(-1,  1,  1),
+        // Back face
+        vec3<f32>( 1, -1, -1), vec3<f32>(-1, -1, -1), vec3<f32>(-1,  1, -1),
+        vec3<f32>( 1, -1, -1), vec3<f32>(-1,  1, -1), vec3<f32>( 1,  1, -1),
+        // Right face
+        vec3<f32>( 1, -1,  1), vec3<f32>( 1, -1, -1), vec3<f32>( 1,  1, -1),
+        vec3<f32>( 1, -1,  1), vec3<f32>( 1,  1, -1), vec3<f32>( 1,  1,  1),
+        // Left face
+        vec3<f32>(-1, -1, -1), vec3<f32>(-1, -1,  1), vec3<f32>(-1,  1,  1),
+        vec3<f32>(-1, -1, -1), vec3<f32>(-1,  1,  1), vec3<f32>(-1,  1, -1),
+        // Top face
+        vec3<f32>(-1,  1,  1), vec3<f32>( 1,  1,  1), vec3<f32>( 1,  1, -1),
+        vec3<f32>(-1,  1,  1), vec3<f32>( 1,  1, -1), vec3<f32>(-1,  1, -1),
+        // Bottom face
+        vec3<f32>(-1, -1, -1), vec3<f32>( 1, -1, -1), vec3<f32>( 1, -1,  1),
+        vec3<f32>(-1, -1, -1), vec3<f32>( 1, -1,  1), vec3<f32>(-1, -1,  1)
+    );
+    return positions[vid];
+}
+
+fn get_cube_normal(vid: u32) -> vec3<f32> {
+    let face_id = vid / 6u;
+    let normals = array<vec3<f32>, 6>(
+        vec3<f32>( 0,  0,  1),  // Front
+        vec3<f32>( 0,  0, -1),  // Back
+        vec3<f32>( 1,  0,  0),  // Right
+        vec3<f32>(-1,  0,  0),  // Left
+        vec3<f32>( 0,  1,  0),  // Top
+        vec3<f32>( 0, -1,  0)   // Bottom
+    );
+    return normals[face_id];
+}
+
+@vertex fn vs_main(@builtin(vertex_index) vid: u32) -> VSOut {
+    let local_pos = get_cube_vertex(vid);
+    let local_normal = get_cube_normal(vid);
+
+    let world_pos = object.model * vec4<f32>(local_pos, 1.0);
+    let world_normal = normalize((object.model * vec4<f32>(local_normal, 0.0)).xyz);
+
+    let clip_pos = uniforms.view_proj * world_pos;
+
+    return VSOut(clip_pos, world_pos.xyz, world_normal);
+}
+
+@fragment fn fs_main(@location(0) world_pos: vec3<f32>, @location(1) normal: vec3<f32>) -> @location(0) vec4<f32> {
+    let N = normalize(normal);
+    let light_dir = normalize(vec3<f32>(1.0, 1.0, 1.0));
+    let diffuse = max(dot(N, light_dir), 0.0);
+
+    let ambient = 0.3;
+    let lighting = ambient + diffuse * 0.7;
+
+    let color = object.color.rgb * lighting;
+    return vec4<f32>(color, 1.0);
+}
diff --git a/workspaces/main/timeline_v2.seq b/workspaces/main/timeline_v2.seq
index 2dc1720..3c97f7b 100644
--- a/workspaces/main/timeline_v2.seq
+++ b/workspaces/main/timeline_v2.seq
@@ -7,10 +7,9 @@ SEQUENCE 0.00 0 "intro"
   EFFECT + PlaceholderEffect temp1 -> sink 0.00 4.00
 
 SEQUENCE 4.00 0 "rotating_cube"
-  # CircleMask (placeholder) -> RotatingCube (placeholder) -> Blur -> sink
-  EFFECT + PlaceholderEffect source -> temp1 0.00 4.00
-  EFFECT + PlaceholderEffect temp1 -> temp2 0.00 4.00
-  EFFECT + GaussianBlurEffect temp2 -> sink 1.00 4.00
+  # RotatingCube -> Blur -> sink
+  EFFECT + RotatingCubeEffectV2 source -> temp1 0.00 4.00
+  EFFECT + GaussianBlurEffect temp1 -> sink 1.00 4.00
 
 SEQUENCE 8.00 0 "flash_cube"
   # FlashCube (placeholder) -> Flash (placeholder) -> sink