🧹 Make Position.UniqueIdentifier an ulong instead of a long

[eduherminio]

If the .NextUInt64() implementation is replaced with (ulong).NextInt64(), no bench changes

Test  | refactor/position-hash-ulong
Elo   | 1.82 +- 2.95 (95%)
SPRT  | 8.0+0.08s Threads=1 Hash=2MB
LLR   | 2.90 (-2.25, 2.89) [-3.00, 1.00]
Games | 23312: +6311 -6189 =10812
Penta | [552, 2795, 4859, 2879, 571]
https://openbench.lynx-chess.com/test/824/

Test  | main
Elo   | 0.44 +- 3.02 (95%)
SPRT  | 8.0+0.08s Threads=1 Hash=2MB
LLR   | 2.92 (-2.25, 2.89) [-5.00, 0.00]
Games | 22322: +6074 -6046 =10202
Penta | [541, 2692, 4683, 2688, 557]
https://openbench.lynx-chess.com/test/825/