feat(uniforms): Add noise field with per-frame random values

Replace _pad with noise field in UniformsSequenceParams, providing
effects with non-deterministic random values [0..1] updated each frame.

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

4 files changed, 9 insertions, 4 deletions

diff --git a/common/shaders/sequence_uniforms.wgsl b/common/shaders/sequence_uniforms.wgsl
index be47ad9..290f89b 100644
--- a/common/shaders/sequence_uniforms.wgsl
+++ b/common/shaders/sequence_uniforms.wgsl
@@ -8,5 +8,5 @@ struct UniformsSequenceParams {
     beat_time: f32,
     beat_phase: f32,
     audio_intensity: f32,
-    _pad: f32,
+    noise: f32,
 };
diff --git a/src/gpu/sequence.cc b/src/gpu/sequence.cc
index 901d560..9de4133 100644
--- a/src/gpu/sequence.cc
+++ b/src/gpu/sequence.cc
@@ -4,6 +4,7 @@
 #include "gpu/effect.h"
 #include "util/fatal_error.h"
 #include <algorithm>
+#include <random>
 
 // NodeRegistry implementation
 
@@ -227,7 +228,11 @@ void Sequence::preprocess(float seq_time, float beat_time, float beat_phase,
   params_.beat_time = beat_time;
   params_.beat_phase = beat_phase;
   params_.audio_intensity = audio_intensity;
-  params_._pad = 0.0f;
+
+  static std::random_device rd;
+  static std::mt19937 gen(rd());
+  static std::uniform_real_distribution<float> dis(0.0f, 1.0f);
+  params_.noise = dis(gen);
 
   uniforms_buffer_.update(ctx_.queue, params_);
 }
diff --git a/src/gpu/sequence.h b/src/gpu/sequence.h
index 3bb770f..2ad0a8f 100644
--- a/src/gpu/sequence.h
+++ b/src/gpu/sequence.h
@@ -38,7 +38,7 @@ struct UniformsSequenceParams {
   float beat_time;  // Musical beats
   float beat_phase; // Fractional beat 0.0-1.0
   float audio_intensity;
-  float _pad;
+  float noise;      // Random value [0..1]
 };
 static_assert(sizeof(UniformsSequenceParams) == 32,
               "UniformsSequenceParams must be 32 bytes for WGSL alignment");
diff --git a/tools/cnn_test.cc b/tools/cnn_test.cc
index 5694a62..1262021 100644
--- a/tools/cnn_test.cc
+++ b/tools/cnn_test.cc
@@ -1391,7 +1391,7 @@ int main(int argc, char** argv) {
         .beat_time = 0.0f,
         .beat_phase = 0.0f,
         .audio_intensity = 0.0f,
-        ._pad = 0.0f,
+        .noise = 0.0f,
     };
     wgpuQueueWriteBuffer(queue, common_uniform_buffer, 0, &common_u,
                          sizeof(common_u));