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, 61 insertions, 15 deletions

diff --git a/cnn_v3/src/cnn_v3_effect.cc b/cnn_v3/src/cnn_v3_effect.cc
index dc26751..e576ceb 100644
--- a/cnn_v3/src/cnn_v3_effect.cc
+++ b/cnn_v3/src/cnn_v3_effect.cc
@@ -187,6 +187,13 @@ CNNv3Effect::CNNv3Effect(const GpuContext& ctx,
   if (weights_data && weights_size == kWeightsBufBytes) {
     upload_weights(ctx_.queue, weights_data, (uint32_t)weights_size);
   }
+
+  size_t mlp_size = 0;
+  const void* mlp_data =
+      GetAsset(AssetId::ASSET_WEIGHTS_CNN_V3_FILM_MLP, &mlp_size);
+  if (mlp_data) {
+    load_film_mlp(mlp_data, (uint32_t)mlp_size);
+  }
 }
 
 // ---------------------------------------------------------------------------
@@ -219,28 +226,67 @@ void CNNv3Effect::upload_weights(WGPUQueue queue, const void* data,
   wgpuQueueWriteBuffer(queue, weights_buf_.buffer, 0, data, size_bytes);
 }
 
+void CNNv3Effect::load_film_mlp(const void* data, uint32_t size_bytes) {
+  if (size_bytes != sizeof(CNNv3FilmMlp)) return;
+  memcpy(&mlp_, data, sizeof(CNNv3FilmMlp));
+  mlp_loaded_ = true;
+}
+
 void CNNv3Effect::set_film_params(const CNNv3FiLMParams& fp) {
-  const float a = fp.audio_intensity;
-  const float b = fp.beat_phase;
+  if (!mlp_loaded_) {
+    // Identity FiLM (γ=1, β=0) — no learned conditioning available.
+    return;
+  }
+
+  // cond[5] = {beat_phase, beat_norm, audio_intensity, style_p0, style_p1}
+  const float cond[5] = {fp.beat_phase, fp.beat_norm, fp.audio_intensity,
+                         fp.style_p0, fp.style_p1};
 
+  // Layer 0: Linear(5→16) + ReLU
+  float h[16];
+  for (int j = 0; j < 16; ++j) {
+    float s = mlp_.l0_b[j];
+    for (int i = 0; i < 5; ++i) s += mlp_.l0_w[j * 5 + i] * cond[i];
+    h[j] = s > 0.f ? s : 0.f;
+  }
+
+  // Layer 1: Linear(16→72)
+  // Output split: g_enc0(8)|b_enc0(8)|g_enc1(16)|b_enc1(16)|g_dec1(8)|b_dec1(8)|g_dec0(4)|b_dec0(4)
+  float film[72];
+  for (int j = 0; j < 72; ++j) {
+    float s = mlp_.l1_b[j];
+    for (int i = 0; i < 16; ++i) s += mlp_.l1_w[j * 16 + i] * h[i];
+    film[j] = s;
+  }
+
+  const float* p = film;
   for (int i = 0; i < 4; ++i) {
-    enc0_params_.gamma_lo[i] = 1.0f + a * 0.5f;
-    enc0_params_.gamma_hi[i] = 1.0f + a * 0.5f;
-    enc0_params_.beta_lo[i]  = b * 0.1f;
-    enc0_params_.beta_hi[i]  = b * 0.1f;
+    enc0_params_.gamma_lo[i] = p[i];
+    enc0_params_.gamma_hi[i] = p[i + 4];
   }
-  for (int i = 0; i < 16; ++i) {
-    enc1_params_.gamma[i] = 1.0f + a * 0.3f;
-    enc1_params_.beta[i]  = fp.beat_norm * 0.1f;
+  p += 8;
+  for (int i = 0; i < 4; ++i) {
+    enc0_params_.beta_lo[i] = p[i];
+    enc0_params_.beta_hi[i] = p[i + 4];
+  }
+  p += 8;
+  for (int i = 0; i < 16; ++i) enc1_params_.gamma[i] = p[i];
+  p += 16;
+  for (int i = 0; i < 16; ++i) enc1_params_.beta[i] = p[i];
+  p += 16;
+  for (int i = 0; i < 4; ++i) {
+    dec1_params_.gamma_lo[i] = p[i];
+    dec1_params_.gamma_hi[i] = p[i + 4];
   }
+  p += 8;
   for (int i = 0; i < 4; ++i) {
-    dec1_params_.gamma_lo[i] = 1.0f + fp.style_p0 * 0.5f;
-    dec1_params_.gamma_hi[i] = 1.0f + fp.style_p0 * 0.5f;
-    dec1_params_.beta_lo[i]  = fp.style_p1 * 0.1f;
-    dec1_params_.beta_hi[i]  = fp.style_p1 * 0.1f;
-    dec0_params_.gamma[i]    = 1.0f + fp.style_p0 * 0.5f;
-    dec0_params_.beta[i]     = fp.style_p1 * 0.1f;
+    dec1_params_.beta_lo[i] = p[i];
+    dec1_params_.beta_hi[i] = p[i + 4];
   }
+  p += 8;
+  for (int i = 0; i < 4; ++i) dec0_params_.gamma[i] = p[i];
+  p += 4;
+  for (int i = 0; i < 4; ++i) dec0_params_.beta[i]  = p[i];
 }
 
 // ---------------------------------------------------------------------------