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, 4 insertions, 0 deletions

diff --git a/src/tests/gpu/test_demo_effects.cc b/src/tests/gpu/test_demo_effects.cc
index 7bba831..0c461b5 100644
--- a/src/tests/gpu/test_demo_effects.cc
+++ b/src/tests/gpu/test_demo_effects.cc
@@ -65,6 +65,10 @@ static void test_effects() {
        std::make_shared<Hybrid3DEffect>(
            fixture.ctx(), std::vector<std::string>{"source"},
            std::vector<std::string>{"sink"})},
+      {"FlashEffect",
+       std::make_shared<FlashEffect>(
+           fixture.ctx(), std::vector<std::string>{"source"},
+           std::vector<std::string>{"sink"})},
   };
 
   int passed = 0;