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>

2 files changed, 121 insertions, 0 deletions

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_;
+};