feat(cnn_v3): add weight assets to assets.txt, update HOW_TO_CNN export docs

- Add WEIGHTS_CNN_V3 and WEIGHTS_CNN_V3_FILM_MLP to workspaces/main/assets.txt
- Add opencv-python and pillow to export_cnn_v3_weights.py uv inline deps
- Update HOW_TO_CNN.md §3 export target → workspaces/main/weights/
- Update HOW_TO_CNN.md §4 weight loading → SafeGetAsset (asset system)

handoff(Gemini): cnn_v3 weight assets registered; export and C++ load path documented

1 files changed, 12 insertions, 14 deletions

diff --git a/cnn_v3/docs/HOW_TO_CNN.md b/cnn_v3/docs/HOW_TO_CNN.md
index 020f79c..458b68f 100644
--- a/cnn_v3/docs/HOW_TO_CNN.md
+++ b/cnn_v3/docs/HOW_TO_CNN.md
@@ -458,11 +458,14 @@ Converts a trained `.pth` checkpoint to two raw binary files for the C++ runtime
 
 ```bash
 cd cnn_v3/training
-python3 export_cnn_v3_weights.py checkpoints/checkpoint_epoch_200.pth
-# writes to export/ by default
-
 python3 export_cnn_v3_weights.py checkpoints/checkpoint_epoch_200.pth \
-    --output /path/to/assets/
+    --output ../../workspaces/main/weights/
+```
+
+Output files are registered in `workspaces/main/assets.txt` as:
+```
+WEIGHTS_CNN_V3, BINARY, weights/cnn_v3_weights.bin, "CNN v3 conv weights (f16, 3928 bytes)"
+WEIGHTS_CNN_V3_FILM_MLP, BINARY, weights/cnn_v3_film_mlp.bin, "CNN v3 FiLM MLP weights (f32, 3104 bytes)"
 ```
 
 ### Output files
@@ -557,20 +560,15 @@ auto cnn = std::make_shared<CNNv3Effect>(
 
 ### Uploading weights
 
-Load `cnn_v3_weights.bin` once at startup, before the first `render()`:
+Load `cnn_v3_weights.bin` once at startup via the asset system, before the first `render()`:
 
 ```cpp
-// Read binary file
-std::vector<uint8_t> data;
-{
-    std::ifstream f("cnn_v3_weights.bin", std::ios::binary | std::ios::ate);
-    data.resize(f.tellg());
-    f.seekg(0);
-    f.read(reinterpret_cast<char*>(data.data()), data.size());
-}
+// Load via asset system
+const char* data = SafeGetAsset(AssetId::ASSET_WEIGHTS_CNN_V3);
+uint32_t size = GetAssetSize(AssetId::ASSET_WEIGHTS_CNN_V3);
 
 // Upload to GPU
-cnn->upload_weights(ctx.queue, data.data(), (uint32_t)data.size());
+cnn->upload_weights(ctx.queue, reinterpret_cast<const uint8_t*>(data), size);
 ```
 
 Before `upload_weights()`: all conv weights are zero, so output is `sigmoid(0) = 0.5` gray.