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, 26 insertions, 0 deletions

diff --git a/src/effects/flash_effect.h b/src/effects/flash_effect.h
new file mode 100644
index 0000000..b9bbff8
--- /dev/null
+++ b/src/effects/flash_effect.h
@@ -0,0 +1,26 @@
+// Flash effect for visual sync testing
+// Pulses white based on beat timing
+
+#pragma once
+#include "gpu/effect.h"
+#include "gpu/uniform_helper.h"
+
+class FlashEffect : public Effect {
+ public:
+  FlashEffect(const GpuContext& ctx,
+              const std::vector<std::string>& inputs,
+              const std::vector<std::string>& outputs);
+  ~FlashEffect() override;
+
+  void render(WGPUCommandEncoder encoder,
+              const UniformsSequenceParams& params,
+              NodeRegistry& nodes) override;
+
+ private:
+  WGPURenderPipeline pipeline_;
+  WGPUBindGroup bind_group_;
+  WGPUSampler sampler_;
+  WGPUTexture dummy_texture_;
+  WGPUTextureView dummy_texture_view_;
+  UniformBuffer<UniformsSequenceParams> uniforms_buffer_;
+};