feat(sequence): port particles_effect to v2

- Add ParticlesEffectV2 with compute + render passes
- Create particle_compute_v2.wgsl and particle_render_v2.wgsl
- Use UniformsSequenceParams for beat-synchronized particles
- Update timeline_v2.seq particles sequence (simplified 2-effect chain)
- Add shader exports to shaders.{h,cc}
- All 36 tests passing

handoff(Claude): Particles v2 complete, rotating_cube next

2 files changed, 84 insertions, 0 deletions

diff --git a/workspaces/main/shaders/particle_compute_v2.wgsl b/workspaces/main/shaders/particle_compute_v2.wgsl
new file mode 100644
index 0000000..3683826
--- /dev/null
+++ b/workspaces/main/shaders/particle_compute_v2.wgsl
@@ -0,0 +1,31 @@
+// Particle simulation (compute shader) - V2
+struct Particle {
+    pos: vec4<f32>,
+    vel: vec4<f32>,
+    rot: vec4<f32>,
+    color: vec4<f32>,
+};
+
+#include "sequence_v2_uniforms"
+
+@group(0) @binding(0) var<storage, read_write> particles: array<Particle>;
+@group(0) @binding(1) var<uniform> uniforms: UniformsSequenceParams;
+
+@compute @workgroup_size(64)
+fn main(@builtin(global_invocation_id) id: vec3<u32>) {
+    let i = id.x;
+    if (i >= arrayLength(&particles)) {
+        return;
+    }
+    var p = particles[i];
+    let new_pos = p.pos.xyz + p.vel.xyz * 0.016;
+    p.pos = vec4<f32>(new_pos, p.pos.w);
+    p.vel.y = p.vel.y - 0.01 * (1.0 + uniforms.audio_intensity * 5.0);
+    p.rot.x = p.rot.x + p.rot.y * 0.016;
+    if (p.pos.y < -1.5) {
+        p.pos.y = 1.5;
+        p.pos.x = (f32(i % 100u) / 50.0) - 1.0 + (uniforms.audio_intensity * 0.5);
+        p.vel.y = 0.0;
+    }
+    particles[i] = p;
+}
diff --git a/workspaces/main/shaders/particle_render_v2.wgsl b/workspaces/main/shaders/particle_render_v2.wgsl
new file mode 100644
index 0000000..8663658
--- /dev/null
+++ b/workspaces/main/shaders/particle_render_v2.wgsl
@@ -0,0 +1,53 @@
+// Particle rendering (vertex + fragment) - V2
+struct Particle {
+    pos: vec4<f32>,
+    vel: vec4<f32>,
+    rot: vec4<f32>,
+    color: vec4<f32>,
+};
+
+#include "sequence_v2_uniforms"
+
+@group(0) @binding(0) var<storage, read> particles: array<Particle>;
+@group(0) @binding(1) var<uniform> uniforms: UniformsSequenceParams;
+
+struct VSOut {
+    @builtin(position) pos: vec4<f32>,
+    @location(0) color: vec4<f32>,
+    @location(1) uv: vec2<f32>,
+};
+
+@vertex fn vs_main(@builtin(vertex_index) vi: u32, @builtin(instance_index) ii: u32) -> VSOut {
+    let p = particles[ii];
+    let size = 0.02 + p.pos.z * 0.01 + uniforms.audio_intensity * 0.02;
+    var offsets = array<vec2<f32>, 6>(
+        vec2<f32>(-1, -1),
+        vec2<f32>(1, -1),
+        vec2<f32>(-1, 1),
+        vec2<f32>(-1, 1),
+        vec2<f32>(1, -1),
+        vec2<f32>(1, 1)
+    );
+    let offset = offsets[vi];
+    let c = cos(p.rot.x);
+    let s = sin(p.rot.x);
+    let rotated_offset = vec2<f32>(offset.x * c - offset.y * s, offset.x * s + offset.y * c);
+    let pos = vec2<f32>(p.pos.x + rotated_offset.x * size / uniforms.aspect_ratio, p.pos.y + rotated_offset.y * size);
+
+    // Fade based on lifetime (p.pos.w goes from 1.0 to 0.0)
+    let lifetime_fade = p.pos.w;
+    let color_with_fade = vec4<f32>(p.color.rgb * (0.5 + 0.5 * uniforms.audio_intensity), p.color.a * lifetime_fade);
+
+    return VSOut(vec4<f32>(pos, 0.0, 1.0), color_with_fade, offset);
+}
+
+@fragment fn fs_main(@location(0) color: vec4<f32>, @location(1) uv: vec2<f32>) -> @location(0) vec4<f32> {
+    // Calculate distance from center for circular shape
+    let dist = length(uv);
+
+    // Smooth circular falloff (1.0 at center, 0.0 at edge)
+    let circle_alpha = smoothstep(1.0, 0.5, dist);
+
+    // Apply circular fade to alpha channel
+    return vec4<f32>(color.rgb, color.a * circle_alpha);
+}