Add Late Move Pruning

[KnightStackSys]

Adds a history-aware Late Move Pruning (LMP) baseline to IronPhoenix for isolated strength testing.

LMP policy

• non-PV nodes only
• disabled while in check
• no depth limit
• quiet moves only
• never prunes TT moves
• never prunes checking moves
• disabled in mate windows
• preserves quiets with history/move-order score >= 2000
• move-count threshold is 3 + depth * depth

Threshold behavior

The threshold still grows with depth, so LMP naturally becomes harder to trigger in deeper nodes even though there is no explicit depth cap.

Examples:

• depth 1: prune after 4 legal moves
• depth 2: prune after 7
• depth 3: prune after 12
• depth 4: prune after 19
• depth 5: prune after 28
• depth 6: prune after 39

Scope

Only src/search.cpp changes. Existing PVS, LMR, SEE pruning, history updates, TT behavior, aspiration windows, and comments are preserved.

Please SPRT this branch against main by itself before combining it with IIR or NMP.