Improve perfs

[garak]

Fix #88

[garak]

Subject	Old	New	Delta	Improvement
benchMove	15.689 us	13.244 us	-2.445 us	15.58% faster
benchMoves	14.473 us	13.461 us	-1.012 us	6.99% faster

[Copilot]

Pull request overview

This PR addresses performance issue #88 by replacing the O(board size) json_encode($this->board)-based position hashing with an incrementally maintained Zobrist-style hash, so repeated makeMove() / undoMove() calls during move generation don’t re-serialize the entire board.

Changes:

• Introduces ZobristHasher and integrates it into Chess to maintain boardHash in O(1) per move/undo.
• Updates generateMoves() cache key construction to avoid ambiguity/collisions (adds delimiters + explicit "all" when $square is null).
• Updates tests to assert deterministic hashing and that side-to-move affects the hash.

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/Chess.php	Swaps board hashing to incremental Zobrist updates on make/undo; refactors cache key format; adds refreshBoardHash().
src/ZobristHasher.php	New hasher implementing rebuild/apply/restore for a 64-bit (8-byte) hash encoded as hex.
src/Entry.php	Stores a previousPosition hash to enable O(1) hash restoration on undo.
tests/MiscTest.php	Updates hash expectations to reflect deterministic Zobrist hashing and side-to-move inclusion.
.gitignore	Updates ignore patterns (root *.cache, adds /build/).

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