Embedded model assets v2

Build-input model assets, SHA-pinned by the repo (crates/cli/assets/MODEL_ASSETS.json + *.sha256 sidecars). Fetched at build time by tools/fetch_model_assets.sh — replaces Git-LFS hosting.

• Qwen3.5-0.8B-MTP-Q4_K_M.gguf: fp32-from-base nano checkpoint (frozen-holdout gates: 99% format-valid, 0 empty; MTP 78.2% brief-dist; eval loss 1.447)
• embeddinggemma-300M-Q4_K.gguf: unchanged from v0.1.x