v1.5.1

Fixes #1: an older coco.nnue in a shared folder could prevent Coco 1.5 from starting.

• Coco now uses its embedded NNUE by default; no separate network file is needed.
• EvalFile defaults to <embedded>. Invalid external-file requests warn and keep the current network.
• The UCI name is now Coco v1.5.1.

Search and network weights are unchanged from v1.5.0. No experimental 1.6 changes are included.