[KnightStackSys]

Adds a conservative Null Move Pruning baseline to IronPhoenix search without changing the existing pruning stack.

Search

• NMP only at non-PV nodes
• disabled while in check
• disabled in mate windows
• minimum depth 4
• dynamic reduction R = 2 + depth / 4
• blocks consecutive null moves
• requires non-pawn material for the current player to reduce zugzwang risk
• synthetic null cutoffs never return manufactured mate scores

Position state

• adds reversible makeNullMove / undoNullMove
• preserves/restores Zobrist key, side-to-move, checkers, castling/alive state, and all 4PC en-passant markers
• expires only the current player's en-passant marker when a null turn is consumed, matching the existing 4PC marker lifecycle

Tests

• property test verifies null move advances one 4PC turn, recomputes the same Zobrist key, and restores the original position exactly on undo

This PR is intentionally isolated for Elo/SPRT testing before merge.