Tune Null Move Pruning for 4PC

[KnightStackSys]

NMP v1 was trending about -18 Elo after ~200 STC games, so this branch keeps the same reversible null-move infrastructure but makes the pruning policy materially more conservative for four-player Teams chess.

NMP v2 tuning

• raise minimum NMP depth from 4 to 5
• require static eval to exceed beta by 100 cp before trying a null search
• slow depth scaling from R = 2 + depth / 4 to R = 2 + depth / 6
• require at least two non-pawn pieces for the side to move
• return the fail-high bound (beta) instead of the synthetic null score, preventing optimistic null scores from inflating parent/aspiration values

Unchanged safety guards

• non-PV only
• disabled in check
• disabled in mate windows
• no consecutive null moves
• no synthetic mate cutoffs

This is intentionally a separate v2 branch so the running v1 STC result stays reproducible. Please SPRT this branch against main with HCE only before merge.