Symbol detector model v1

First trained detector: torchvision Faster R-CNN MobileNetV3-FPN, 30 epochs on 2,000 synthetic sheets (56,293 instances, seed 42). Apache-2.0 training stack. 28 detections above 0.35 on the first held-out sheet. Hosted here as a release asset because the file exceeds Cloudflare Pages' 25MB per-file limit; the app fetches it cross-origin (download-only, same as the ONNX runtime wasm).