Add Mate Distance Pruning

[KnightStackSys]

Adds Mate Distance Pruning (MDP) as an isolated search experiment.

Policy

• clamps alpha to the worst mate score still reachable at the current ply
• clamps beta to the best mate score still reachable at the current ply
• immediately returns when those mate bounds collapse the window
• applies in both the regular negamax search and qsearch

IronPhoenix score convention

IronPhoenix scores an immediate terminal king capture as MATE_SCORE - (ply + 1) and a side already mated at the current node as -MATE_SCORE + ply. The MDP bounds are aligned to those exact conventions instead of copying a generic two-player formula.

Scope

• only src/search.cpp changes
• no changes to LMR, LMP, SEE pruning, TT policy, aspiration windows, move ordering, history, or evaluation

This branch is intentionally isolated for SPRT testing directly against main.

[KnightStackSys]

No SPRT needed for this PR