Add reverse futility pruning

[KnightStackSys]

Adds an isolated Reverse Futility Pruning (RFP) experiment on top of current main.

Behavior

• non-PV nodes only
• never at root (ply > 0)
• disabled while the side to move is in check
• disabled while the teammate is in check
• disabled in mate-score windows
• requires non-pawn material
• limited to depth <= 4
• uses a depth-scaled margin of 200 * depth
• cuts off when staticEval - margin >= beta
• never returns a synthetic mate score

NMP interaction

RFP and existing null-move pruning share the same static evaluation and partner-check result when both are eligible, avoiding duplicate evaluator/check work. Existing NMP thresholds and reduction formula are unchanged.

Scope

• src/search.cpp only
• no changes to LMP, SEE pruning, LMR, evaluation, move ordering, TT, or board representation

This is a search-strength experiment. SPRT directly against main before merge.

[KnightStackSys]

RFP + Post LMR continuation histories