A full 64-bit position key (native ints), folding all piece bitboards plus
side-to-move, castling rights and the en-passant square. Cheap — no FEN
rebuild — and collisions are negligible for a per-search table.
Pass the turn to the opponent without moving (null-move pruning). Only the
side to move and en-passant square change; the board is untouched. Never
call when inCheck (the null side could be mated).
Static Exchange Evaluation: the material a capture move wins or loses
after the full sequence of optimal recaptures on the target square, in
centipawns from the moving side's view. Negative means a losing capture.