Feature - Integrate a basic opening book

[Codex-Crusader]

Summary

Add support for a basic opening book to guide the engine in early moves.

Motivation

An opening book improves early-game play and can speed training/evaluation by avoiding random early-game noise.

Acceptance criteria / Definition of Done

• Add an opening_book module that:
  • Loads a simple book format (JSON / polyglot) placed under data/openings/.
  • Provides a function get_book_move(board) that returns a move if available.
• Engine consults the opening book for the first N plies (configurable).
• Document how to add / update the opening book and include a small sample book.

Implementation notes

• Start with a simple JSON mapping fen -> [move1, move2, ...] or a tiny polyglot file.
• Keep usage optional (configurable from config.yaml).

Complexity

Medium — straightforward but requires careful integration.