fix(cnn_v3): remove dec0 ReLU, load FiLM MLP at runtime

Two bugs blocking training convergence:

1. dec0 ReLU before sigmoid constrained output to [0.5,1.0] — network
   could never produce dark pixels. Removed F.relu in train_cnn_v3.py
   and max(0,…) in cnn_v3_dec0.wgsl. Test vectors regenerated.

2. set_film_params() used hardcoded heuristics instead of the trained MLP.
   Added CNNv3FilmMlp struct + load_film_mlp() to cnn_v3_effect.h/.cc.
   MLP auto-loaded from ASSET_WEIGHTS_CNN_V3_FILM_MLP at construction;
   Linear(5→16)→ReLU→Linear(16→72) runs CPU-side each frame.

36/36 tests pass. Parity max_err=4.88e-4 unchanged.

handoff(Gemini): retrain from scratch — needs ≥50 samples (currently 11).
See cnn_v3/docs/HOWTO.md §2-3.

1 files changed, 20 insertions, 2 deletions

diff --git a/cnn_v3/src/cnn_v3_effect.h b/cnn_v3/src/cnn_v3_effect.h
index 070f988..589680c 100644
--- a/cnn_v3/src/cnn_v3_effect.h
+++ b/cnn_v3/src/cnn_v3_effect.h
@@ -7,7 +7,7 @@
 //   enc1:       Conv(8→16,  3×3) + FiLM16 + ReLU       H/2×W/2  2× rgba32uint
 //   bottleneck: Conv(16→16, 3×3, dil=2)   + ReLU       H/4×W/4  2× rgba32uint
 //   dec1:       Conv(32→8,  3×3) + FiLM8  + ReLU       H/2×W/2  rgba32uint
-//   dec0:       Conv(16→4,  3×3) + FiLM4  + ReLU + sig H×W      rgba16float
+//   dec0:       Conv(16→4,  3×3) + FiLM4  + sig   H×W      rgba16float
 //
 // Inputs:  feat_tex0, feat_tex1   (rgba32uint, 20-channel G-buffer)
 // Output:  output_tex             (rgba16float, 4-channel RGBA)
@@ -97,6 +97,17 @@ struct CNNv3FiLMParams {
   float style_p1        = 0.0f;  // user-defined style param
 };
 
+// FiLM MLP weights: Linear(5→16)→ReLU→Linear(16→72).
+// Loaded from cnn_v3_film_mlp.bin (1320 f32 = 5280 bytes).
+// Layout: l0_w(80) | l0_b(16) | l1_w(1152) | l1_b(72), all row-major f32.
+struct CNNv3FilmMlp {
+  float l0_w[16 * 5];   // (16, 5) row-major
+  float l0_b[16];
+  float l1_w[72 * 16];  // (72, 16) row-major
+  float l1_b[72];
+};
+static_assert(sizeof(CNNv3FilmMlp) == 1320 * 4, "CNNv3FilmMlp size mismatch");
+
 class CNNv3Effect : public Effect {
  public:
   CNNv3Effect(const GpuContext& ctx, const std::vector<std::string>& inputs,
@@ -111,9 +122,13 @@ class CNNv3Effect : public Effect {
   // Update FiLM conditioning; call before render() each frame.
   void set_film_params(const CNNv3FiLMParams& fp);
 
-  // Upload packed-f16 weights (kWeightsBufBytes bytes of u32 pairs).
+  // Upload packed-f16 conv weights (kWeightsBufBytes bytes of u32 pairs).
   void upload_weights(WGPUQueue queue, const void* data, uint32_t size_bytes);
 
+  // Load FiLM MLP weights from cnn_v3_film_mlp.bin (1320 f32 = 5280 bytes).
+  // Must be called before set_film_params() for learned conditioning.
+  void load_film_mlp(const void* data, uint32_t size_bytes);
+
  private:
   // Intermediate node names (prefixed from output[0])
   std::string node_enc0_;
@@ -156,4 +171,7 @@ class CNNv3Effect : public Effect {
 
   void create_pipelines();
   void update_bind_groups(NodeRegistry& nodes);
+
+  CNNv3FilmMlp mlp_{};
+  bool mlp_loaded_ = false;
 };