⚡ Use Unsafe.Add for PSQT()

[eduherminio]

• Use Unsafe.Add to properly avoid bound checks when querying PSQTs (if we're going unsafe, let's do it 'right' in the most performant way)
• Add Assert to for manually verify bounds in debug mode

Part of planned https://github.com/lynx-chess/Backlog/issues/300, and was brought up again by @tannergooding in C# Discord while reviewing unsafe usage 😻

Test  | perft/psqt-unsafe-add
Elo   | 4.30 +- 4.08 (95%)
SPRT  | 8.0+0.08s Threads=1 Hash=32MB
LLR   | 2.95 (-2.25, 2.89) [-3.00, 1.00]
Games | 11646: +3304 -3160 =5182
Penta | [259, 1294, 2604, 1376, 290]
https://openbench.lynx-chess.com/test/935/

[tannergooding]

notably the unsafe { } is unnecessary