fix: Resolve CNN effect black screen bug (framebuffer capture + uniforms)

Two bugs causing black screen when CNN post-processing activated:

1. Framebuffer capture timing: Capture ran inside post-effect loop after
   ping-pong swaps, causing layers 1+ to capture wrong buffer. Moved
   capture before loop to copy framebuffer_a once before post-chain starts.

2. Missing uniforms update: CNNEffect never updated uniforms_ buffer,
   leaving uniforms.resolution uninitialized (0,0). UV calculation
   p.xy/uniforms.resolution produced NaN, causing all texture samples
   to return black. Added uniforms update in update_bind_group().

Files modified:
- src/gpu/effect.cc: Capture before post-chain (lines 308-346)
- src/gpu/effects/cnn_effect.cc: Add uniforms update (lines 132-142)
- workspaces/main/shaders/cnn/cnn_layer.wgsl: Remove obsolete comment
- doc/CNN_DEBUG.md: Historical debugging doc
- CLAUDE.md: Reference CNN_DEBUG.md in historical section

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

2 files changed, 53 insertions, 34 deletions

diff --git a/src/gpu/effect.cc b/src/gpu/effect.cc
index b50acce..fba3353 100644
--- a/src/gpu/effect.cc
+++ b/src/gpu/effect.cc
@@ -293,6 +293,45 @@ void MainSequence::render_frame(float global_time, float beat, float peak,
   wgpuRenderPassEncoderEnd(scene_pass);
 
   // 3. Post Chain
+
+  // Capture framebuffer ONCE before post-processing chain
+  bool needs_capture = false;
+  for (const SequenceItem* item : post_effects) {
+    PostProcessEffect* pp = (PostProcessEffect*)(item->effect.get());
+    if (pp->needs_framebuffer_capture()) {
+      needs_capture = true;
+      break;
+    }
+  }
+
+  if (needs_capture) {
+    WGPUTextureView captured_view = get_auxiliary_view("captured_frame");
+    if (captured_view) {
+      WGPURenderPassColorAttachment capture_attachment = {};
+      capture_attachment.view = captured_view;
+      capture_attachment.resolveTarget = nullptr;
+      capture_attachment.loadOp = WGPULoadOp_Clear;
+      capture_attachment.storeOp = WGPUStoreOp_Store;
+      capture_attachment.clearValue = {0.0f, 0.0f, 0.0f, 1.0f};
+#if !defined(DEMO_CROSS_COMPILE_WIN32)
+      capture_attachment.depthSlice = WGPU_DEPTH_SLICE_UNDEFINED;
+#endif
+      WGPURenderPassDescriptor capture_desc = {
+          .colorAttachmentCount = 1, .colorAttachments = &capture_attachment};
+      WGPURenderPassEncoder capture_pass =
+          wgpuCommandEncoderBeginRenderPass(encoder, &capture_desc);
+      wgpuRenderPassEncoderSetViewport(capture_pass, 0.0f, 0.0f,
+                                       (float)width_, (float)height_, 0.0f, 1.0f);
+
+      PostProcessEffect* passthrough =
+          (PostProcessEffect*)passthrough_effect_.get();
+      passthrough->update_bind_group(framebuffer_view_a_);
+      passthrough->render(capture_pass, 0, 0, 0, aspect_ratio);
+
+      wgpuRenderPassEncoderEnd(capture_pass);
+    }
+  }
+
   WGPUSurfaceTexture st = {};
   WGPUTextureView final_view = nullptr;
 
@@ -340,39 +379,6 @@ void MainSequence::render_frame(float global_time, float beat, float peak,
       PostProcessEffect* pp =
           (PostProcessEffect*)(post_effects[i]->effect.get());
 
-      // Capture framebuffer if effect needs it
-      if (pp->needs_framebuffer_capture()) {
-        WGPUTextureView captured_view = get_auxiliary_view("captured_frame");
-        if (captured_view) {
-          // Get source texture from current_input view
-          // Note: This is a simplified blit using a render pass
-          WGPURenderPassColorAttachment capture_attachment = {};
-          capture_attachment.view = captured_view;
-          capture_attachment.resolveTarget = nullptr;
-          capture_attachment.loadOp = WGPULoadOp_Clear;
-          capture_attachment.storeOp = WGPUStoreOp_Store;
-          capture_attachment.clearValue = {0.0f, 0.0f, 0.0f, 1.0f};
-#if !defined(DEMO_CROSS_COMPILE_WIN32)
-          capture_attachment.depthSlice = WGPU_DEPTH_SLICE_UNDEFINED;
-#endif
-          WGPURenderPassDescriptor capture_desc = {
-              .colorAttachmentCount = 1, .colorAttachments = &capture_attachment};
-          WGPURenderPassEncoder capture_pass =
-              wgpuCommandEncoderBeginRenderPass(encoder, &capture_desc);
-          wgpuRenderPassEncoderSetViewport(capture_pass, 0.0f, 0.0f,
-                                           (float)width_, (float)height_, 0.0f,
-                                           1.0f);
-
-          // Use passthrough effect to copy framebuffer_a (scene) to captured_frame
-          PostProcessEffect* passthrough =
-              (PostProcessEffect*)passthrough_effect_.get();
-          passthrough->update_bind_group(framebuffer_view_a_);
-          passthrough->render(capture_pass, 0, 0, 0, aspect_ratio);
-
-          wgpuRenderPassEncoderEnd(capture_pass);
-        }
-      }
-
       pp->update_bind_group(current_input);
 
       WGPURenderPassColorAttachment pp_attachment = {};
diff --git a/src/gpu/effects/cnn_effect.cc b/src/gpu/effects/cnn_effect.cc
index cb00455..7107bea 100644
--- a/src/gpu/effects/cnn_effect.cc
+++ b/src/gpu/effects/cnn_effect.cc
@@ -104,7 +104,10 @@ void CNNEffect::init(MainSequence* demo) {
 
 void CNNEffect::render(WGPURenderPassEncoder pass, float time, float beat,
                       float intensity, float aspect_ratio) {
-  if (!bind_group_) return;
+  if (!bind_group_) {
+    fprintf(stderr, "CNN render: no bind_group\n");
+    return;
+  }
 
   wgpuRenderPassEncoderSetPipeline(pass, pipeline_);
   wgpuRenderPassEncoderSetBindGroup(pass, 0, bind_group_, 0, nullptr);
@@ -114,6 +117,16 @@ void CNNEffect::render(WGPURenderPassEncoder pass, float time, float beat,
 void CNNEffect::update_bind_group(WGPUTextureView input_view) {
   input_view_ = input_view;
 
+  // Update common uniforms (CRITICAL for UV calculation!)
+  const CommonPostProcessUniforms u = {
+      .resolution = {(float)width_, (float)height_},
+      .aspect_ratio = (float)width_ / (float)height_,
+      .time = 0.0f,
+      .beat = 0.0f,
+      .audio_intensity = 0.0f,
+  };
+  uniforms_.update(ctx_.queue, u);
+
   // All layers: get captured frame (original input from layer 0)
   if (demo_) {
     original_view_ = demo_->get_auxiliary_view("captured_frame");