Add history move pruning

[KnightStackSys]

Adds an isolated History Move Pruning (HMP) experiment on top of the current main.

Behavior

• applies only at non-PV nodes
• applies only when the side to move is not in check
• quiet moves only; captures and promotions are excluded
• never prunes the TT move
• never prunes checking moves
• never prunes mate-window moves
• protects the first legal searched move
• limited to depth <= 4
• uses a depth-scaled negative history threshold: -1500 * depth

Placement

HMP runs after existing Late Move Pruning and before SEE pruning.

Scope

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

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