feat: Add PeakMeterEffect v2 for test_demo audio visualization

Ports PeakMeterEffect to v2 Effect system with proper DAG routing.
Red horizontal bar overlay displays audio_intensity for visual debugging
of audio-visual synchronization.

Changes:
- New: src/effects/peak_meter_effect.{h,cc} - v2 implementation
- Timeline: FlashEffect -> flash_out -> PeakMeterEffect -> sink
- Build: Added to COMMON_GPU_EFFECTS and demo_effects.h
- Test: Added to test_demo_effects.cc (9/9 effects pass)
- Cleanup: Removed old disabled PeakMeterEffect code from test_demo.cc

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

7 files changed, 129 insertions, 90 deletions

diff --git a/cmake/DemoSourceLists.cmake b/cmake/DemoSourceLists.cmake
index a532a9a..3c162e9 100644
--- a/cmake/DemoSourceLists.cmake
+++ b/cmake/DemoSourceLists.cmake
@@ -38,6 +38,7 @@ set(COMMON_GPU_EFFECTS
     src/effects/rotating_cube_effect.cc
     src/effects/hybrid3_d_effect.cc
     src/effects/flash_effect.cc
+    src/effects/peak_meter_effect.cc
     # TODO: Port CNN effects to v2 (complex v1 dependencies)
     # cnn_v1/src/cnn_v1_effect.cc
     # cnn_v2/src/cnn_v2_effect.cc
diff --git a/src/app/test_demo.cc b/src/app/test_demo.cc
index 83e6f8b..6031aef 100644
--- a/src/app/test_demo.cc
+++ b/src/app/test_demo.cc
@@ -19,89 +19,6 @@
 #include "generated/test_timeline.h"
 extern float GetDemoDuration();
 
-// TODO: Port PeakMeterEffect and CNN effects to v2
-// #include "../../cnn_v1/src/cnn_v1_effect.h"
-// #include "../../cnn_v2/src/cnn_v2_effect.h"
-// #include "gpu/post_process_helper.h"
-// #include "gpu/shader_composer.h"
-
-#if 0 // Disabled: needs v2 port
-class PeakMeterEffect : public PostProcessEffectV2 {
- public:
-  PeakMeterEffect(const GpuContext& ctx) : PostProcessEffect(ctx) {
-    // Use ShaderComposer to include CommonUniforms from common_uniforms.wgsl
-    const char* shader_main = R"(
-      struct VertexOutput {
-        @builtin(position) position: vec4<f32>,
-        @location(0) uv: vec2<f32>,
-      };
-
-      struct EffectParams {
-        unused: f32,
-      };
-
-      @group(0) @binding(0) var inputSampler: sampler;
-      @group(0) @binding(1) var inputTexture: texture_2d<f32>;
-      @group(0) @binding(2) var<uniform> uniforms: CommonUniforms;
-      @group(0) @binding(3) var<uniform> params: EffectParams;
-
-      @vertex
-      fn vs_main(@builtin(vertex_index) vertexIndex: u32) -> VertexOutput {
-        var output: VertexOutput;
-        // Full-screen triangle (required for post-process pass-through)
-        var pos = array<vec2<f32>, 3>(
-          vec2<f32>(-1.0, -1.0),
-          vec2<f32>(3.0, -1.0),
-          vec2<f32>(-1.0, 3.0)
-        );
-        output.position = vec4<f32>(pos[vertexIndex], 0.0, 1.0);
-        output.uv = pos[vertexIndex] * 0.5 + 0.5;
-        return output;
-      }
-
-      @fragment
-      fn fs_main(input: VertexOutput) -> @location(0) vec4<f32> {
-        // Bar dimensions
-        let bar_y_min = 0.005;
-        let bar_y_max = 0.015;
-        let bar_x_min = 0.015;
-        let bar_x_max = 0.250;
-        let in_bar_y = input.uv.y >= bar_y_min && input.uv.y <= bar_y_max;
-        let in_bar_x = input.uv.x >= bar_x_min && input.uv.x <= bar_x_max;
-
-        // Optimization: Return bar color early (avoids texture sampling for ~5% of pixels)
-        if (in_bar_y && in_bar_x) {
-          let uv_x = (input.uv.x - bar_x_min) / (bar_x_max - bar_x_min);
-          let factor = step(uv_x, uniforms.audio_intensity);
-          return mix(vec4<f32>(0.0, 0.0, 0.0, 1.0), vec4<f32>(1.0, 0.0, 0.0,1.0), factor);
-        }
-
-        // Pass through input texture for rest of screen
-        return textureSample(inputTexture, inputSampler, input.uv);
-      }
-    )";
-
-    // Compose shader with common_uniforms to get CommonUniforms definition
-    std::string shader_code =
-        ShaderComposer::Get().Compose({"common_uniforms"}, shader_main);
-
-    pipeline_ = create_post_process_pipeline(ctx_.device, ctx_.format,
-                                             shader_code.c_str());
-  }
-
-  void update_bind_group(WGPUTextureView input_view) override {
-    pp_update_bind_group(ctx_.device, pipeline_, &bind_group_, input_view,
-                         uniforms_.get(), {});
-  }
-
-  void render(WGPURenderPassEncoder pass,
-              const UniformsSequenceParams& uniforms) override {
-    uniforms_.update(ctx_.queue, uniforms);
-    PostProcessEffect::render(pass, uniforms);
-  }
-};
-#endif // Disabled: needs v2 port
-
 static int g_cnn_version = 2; // Default to v2
 
 #if !defined(STRIP_ALL)
@@ -228,12 +145,6 @@ int main(int argc, char** argv) {
   // Initialize v2 timeline from test_demo.seq
   InitializeSequences(*gpu_get_context(), width, height);
 
-#if !defined(STRIP_ALL)
-  // TODO: Port CNN and peak meter effects to v2
-  // const GpuContext* gpu_ctx = gpu_get_context();
-  // (v1 gpu_add_custom_effect not available in v2)
-#endif
-
   audio_init();
 
   static AudioEngine g_audio_engine;
diff --git a/src/effects/peak_meter_effect.cc b/src/effects/peak_meter_effect.cc
new file mode 100644
index 0000000..692d851
--- /dev/null
+++ b/src/effects/peak_meter_effect.cc
@@ -0,0 +1,98 @@
+// Peak meter overlay for audio debugging
+
+#include "effects/peak_meter_effect.h"
+#include "gpu/post_process_helper.h"
+#include "gpu/shader_composer.h"
+#include "util/fatal_error.h"
+
+PeakMeterEffect::PeakMeterEffect(const GpuContext& ctx,
+                                 const std::vector<std::string>& inputs,
+                                 const std::vector<std::string>& outputs)
+    : Effect(ctx, inputs, outputs), pipeline_(nullptr), bind_group_(nullptr) {
+  HEADLESS_RETURN_IF_NULL(ctx_.device);
+
+  uniforms_buffer_.init(ctx_.device);
+
+  const char* shader_main = R"(
+    struct VertexOutput {
+      @builtin(position) position: vec4<f32>,
+      @location(0) uv: vec2<f32>,
+    };
+
+    @group(0) @binding(0) var inputSampler: sampler;
+    @group(0) @binding(1) var inputTexture: texture_2d<f32>;
+    @group(0) @binding(2) var<uniform> uniforms: CommonUniforms;
+
+    @vertex
+    fn vs_main(@builtin(vertex_index) vertexIndex: u32) -> VertexOutput {
+      var output: VertexOutput;
+      var pos = array<vec2<f32>, 3>(
+        vec2<f32>(-1.0, -1.0),
+        vec2<f32>(3.0, -1.0),
+        vec2<f32>(-1.0, 3.0)
+      );
+      output.position = vec4<f32>(pos[vertexIndex], 0.0, 1.0);
+      output.uv = pos[vertexIndex] * 0.5 + 0.5;
+      return output;
+    }
+
+    @fragment
+    fn fs_main(input: VertexOutput) -> @location(0) vec4<f32> {
+      let bar_y_min = 0.005;
+      let bar_y_max = 0.015;
+      let bar_x_min = 0.015;
+      let bar_x_max = 0.250;
+      let in_bar_y = input.uv.y >= bar_y_min && input.uv.y <= bar_y_max;
+      let in_bar_x = input.uv.x >= bar_x_min && input.uv.x <= bar_x_max;
+
+      if (in_bar_y && in_bar_x) {
+        let uv_x = (input.uv.x - bar_x_min) / (bar_x_max - bar_x_min);
+        let factor = step(uv_x, uniforms.audio_intensity);
+        return mix(vec4<f32>(0.0, 0.0, 0.0, 1.0), vec4<f32>(1.0, 0.0, 0.0, 1.0), factor);
+      }
+
+      return textureSample(inputTexture, inputSampler, input.uv);
+    }
+  )";
+
+  std::string shader_code =
+      ShaderComposer::Get().Compose({"common_uniforms"}, shader_main);
+
+  pipeline_ = create_post_process_pipeline(ctx_.device,
+                                           WGPUTextureFormat_RGBA8Unorm,
+                                           shader_code.c_str());
+}
+
+PeakMeterEffect::~PeakMeterEffect() {
+  if (bind_group_) wgpuBindGroupRelease(bind_group_);
+  if (pipeline_) wgpuRenderPipelineRelease(pipeline_);
+}
+
+void PeakMeterEffect::render(WGPUCommandEncoder encoder,
+                             const UniformsSequenceParams& params,
+                             NodeRegistry& nodes) {
+  WGPUTextureView input_view = nodes.get_view(input_nodes_[0]);
+  WGPUTextureView output_view = nodes.get_view(output_nodes_[0]);
+
+  uniforms_buffer_.update(ctx_.queue, params);
+  pp_update_bind_group(ctx_.device, pipeline_, &bind_group_, input_view,
+                       uniforms_buffer_.get(), {nullptr, 0});
+
+  WGPURenderPassColorAttachment color_attachment = {};
+  color_attachment.view = output_view;
+  color_attachment.depthSlice = WGPU_DEPTH_SLICE_UNDEFINED;
+  color_attachment.loadOp = WGPULoadOp_Load;
+  color_attachment.storeOp = WGPUStoreOp_Store;
+  color_attachment.clearValue = {0.0, 0.0, 0.0, 1.0};
+
+  WGPURenderPassDescriptor pass_desc = {};
+  pass_desc.colorAttachmentCount = 1;
+  pass_desc.colorAttachments = &color_attachment;
+
+  WGPURenderPassEncoder pass = wgpuCommandEncoderBeginRenderPass(encoder, &pass_desc);
+  wgpuRenderPassEncoderSetPipeline(pass, pipeline_);
+  wgpuRenderPassEncoderSetBindGroup(pass, 0, bind_group_, 0, nullptr);
+  wgpuRenderPassEncoderDraw(pass, 3, 1, 0, 0);
+  wgpuRenderPassEncoderEnd(pass);
+  wgpuRenderPassEncoderRelease(pass);
+}
diff --git a/src/effects/peak_meter_effect.h b/src/effects/peak_meter_effect.h
new file mode 100644
index 0000000..905bcda
--- /dev/null
+++ b/src/effects/peak_meter_effect.h
@@ -0,0 +1,23 @@
+// Peak meter overlay for audio debugging
+// Renders red horizontal bar showing audio_intensity
+
+#pragma once
+#include "gpu/effect.h"
+#include "gpu/uniform_helper.h"
+
+class PeakMeterEffect : public Effect {
+ public:
+  PeakMeterEffect(const GpuContext& ctx,
+                  const std::vector<std::string>& inputs,
+                  const std::vector<std::string>& outputs);
+  ~PeakMeterEffect() override;
+
+  void render(WGPUCommandEncoder encoder,
+              const UniformsSequenceParams& params,
+              NodeRegistry& nodes) override;
+
+ private:
+  WGPURenderPipeline pipeline_;
+  WGPUBindGroup bind_group_;
+  UniformBuffer<UniformsSequenceParams> uniforms_buffer_;
+};
diff --git a/src/gpu/demo_effects.h b/src/gpu/demo_effects.h
index 0151b80..999deb1 100644
--- a/src/gpu/demo_effects.h
+++ b/src/gpu/demo_effects.h
@@ -22,6 +22,7 @@
 #include "effects/hybrid3_d_effect.h"
 #include "effects/particles_effect.h"
 #include "effects/passthrough_effect.h"
+#include "effects/peak_meter_effect.h"
 #include "effects/placeholder_effect.h"
 #include "effects/rotating_cube_effect.h"
 #include "effects/flash_effect.h"
diff --git a/src/tests/gpu/test_demo_effects.cc b/src/tests/gpu/test_demo_effects.cc
index 0c461b5..cebf4a6 100644
--- a/src/tests/gpu/test_demo_effects.cc
+++ b/src/tests/gpu/test_demo_effects.cc
@@ -69,6 +69,10 @@ static void test_effects() {
        std::make_shared<FlashEffect>(
            fixture.ctx(), std::vector<std::string>{"source"},
            std::vector<std::string>{"sink"})},
+      {"PeakMeterEffect",
+       std::make_shared<PeakMeterEffect>(
+           fixture.ctx(), std::vector<std::string>{"source"},
+           std::vector<std::string>{"sink"})},
   };
 
   int passed = 0;
diff --git a/workspaces/test/timeline.seq b/workspaces/test/timeline.seq
index 8907014..aa97580 100644
--- a/workspaces/test/timeline.seq
+++ b/workspaces/test/timeline.seq
@@ -3,6 +3,7 @@
 # BPM 120 (set in test_demo.track)
 
 SEQUENCE 0.0 0 "MainLoop"
-EFFECT + FlashEffect source -> sink 0.0 16.0
+EFFECT + FlashEffect source -> flash_out 0.0 16.0
+EFFECT + PeakMeterEffect flash_out -> sink 0.0 16.0
 
 END_DEMO 32.0