🔍 Make TranspositionTableElement.Key an ushort instead of a short

[eduherminio]

This might prevent information loss or the attempt to use the wrong TT entries, given how (short)n can be -1 instead of the intended value it'd have with (ushort)n.

Test  | tt-key-ushort
Elo   | 4.68 +- 5.09 (95%)
SPRT  | 8.0+0.08s Threads=1 Hash=4MB
LLR   | 2.93 (-2.25, 2.89) [-5.00, 0.00]
Games | 7130: +2036 -1940 =3154
Penta | [144, 772, 1662, 818, 169]
https://openbench.lynx-chess.com/test/799/