feat: Add FlashEffect for audio/visual sync testing

- FlashEffect: Beat-synchronized white flash using ShaderComposer
- Loads shader from assets (flash.wgsl) with sequence_uniforms include
- Uses pow(1.0 - beat_phase, 4.0) for sharp flash at beat start
- Updated test_demo.seq to use FlashEffect (was HeptagonEffect)
- Added FlashEffect to test suite (test_demo_effects.cc)
- Made cnn_test conditional on main workspace (fixes build error)
- Flash intensity: 1.0 at beat start, fades to 0.0 by beat end

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

1 files changed, 27 insertions, 0 deletions

diff --git a/workspaces/test/shaders/flash.wgsl b/workspaces/test/shaders/flash.wgsl
new file mode 100644
index 0000000..8f8c64c
--- /dev/null
+++ b/workspaces/test/shaders/flash.wgsl
@@ -0,0 +1,27 @@
+// Flash shader for visual sync testing
+// Pulses white on beat
+#include "sequence_uniforms"
+
+@group(0) @binding(2) var<uniform> uniforms: UniformsSequenceParams;
+
+struct VertexOutput {
+    @builtin(position) position: vec4<f32>,
+    @location(0) uv: vec2<f32>,
+};
+
+@vertex fn vs_main(@builtin(vertex_index) vid: u32) -> VertexOutput {
+    var out: VertexOutput;
+    let x = f32((vid & 1u) << 1u);
+    let y = f32((vid & 2u));
+    out.position = vec4<f32>(x * 2.0 - 1.0, 1.0 - y * 2.0, 0.0, 1.0);
+    out.uv = vec2<f32>(x, y);
+    return out;
+}
+
+@fragment fn fs_main(in: VertexOutput) -> @location(0) vec4<f32> {
+  // Strong flash on beat, fades quickly
+  let intensity = pow(1.0 - uniforms.beat_phase, 4.0);
+  // Add audio intensity for extra punch
+  let final_intensity = intensity * (1.0 + uniforms.audio_intensity * 0.5);
+  return vec4<f32>(final_intensity, final_intensity, final_intensity, 1.0);
+}