feat: Add complete Chess960 (Fischer Random) support with UCI convention

[fafriat]

Summary

Adds full Chess960 castling support to chesslib, including move generation, validation, execution, undo, SAN/UCI parsing, and FEN round-trip. All 960 starting positions are handled correctly.

Closes #122

Changes

Detection

• Board.loadFromFen() auto-detects Chess960 from Shredder-FEN (AHah) or KQkq with king not on e-file
• Board.loadFromFen(fen, true) forces Chess960 mode (useful when PGN has [Variant "Chess960"])

GameContext

• loadChess960() configures king/rook moves, traversed squares, and "must be empty" bitboards dynamically from actual piece positions

Move execution

• Board.doMove() handles Chess960 castling with manual piece placement (avoids capture issues when king/rook are adjacent)
• Board.isMoveLegal() excludes king/rook from occupancy checks for castling validation
• MoveBackup.restore() handles Chess960 castle undo

Move generation

• MoveGenerator.generateCastleMoves() excludes king/rook from occupancy checks

UCI support

• Board.toUci(Move) returns king→rook notation for Chess960 castling (e.g. g1h1), following the official UCI Chess960 convention
• Board.fromUci(String) parses king→rook UCI notation as castling
• MoveList.loadFromText() uses board.fromUci() for correct Chess960 UCI parsing

SAN support

• MoveList.encode() uses context.isCastleMove() instead of file-delta for castle detection (handles king moves of 0-1 files in Chess960)

FEN

• Board.getFen() outputs Shredder-FEN castling rights for Chess960 positions

Edge cases handled

• King already on destination (e.g. king on g1 doing O-O)
• King and rook adjacent or swapping squares
• Rook between king start and king destination
• Castle rights properly lost when rook or king moves

Tests

14 new tests covering detection, castling O-O/O-O-O, undo, SAN parsing, UCI conversion, loadFromText, FEN round-trip, standard chess unaffected, and castle rights loss.

All 125 existing tests pass unchanged.