[KnightStackSys]

Summary

• Implement forward futility pruning in the negamax move loop (separate from existing node-level reverse futility pruning)
• Use optimistic bound staticEval + futilityBaseMargin + depth * futilityMarginPerDepth <= alpha
• Prune only late quiet, non-checking moves after at least one legal move, outside PV, root, checks (including teammate check), mate windows and null subtrees
• Reuse search stack's cached evaluation and check only genuinely plausible pruning candidates
• Add SearchFutilityMaxDepth (default 2; 0 disables), SearchFutilityBaseMargin (100), and SearchFutilityMarginPerDepth (150) as UCI-tunable options
• Add dedicated ironphoenix_futility regression test (UCI, node-count effect, exclusions), documentation

Notes

• Includes current Lazy SMP and existing tuned Search/Eval defaults. Does not merge IIR.
• CI compile/tests then paired SPRT vs SearchFutilityMaxDepth=0 recommended before merging.