Add Internal Iterative Reduction

[KnightStackSys]

Adds a conservative Internal Iterative Reduction (IIR) baseline to IronPhoenix search for isolated Elo testing.

IIR policy

• applies only at PV nodes
• skips the root (ply > 0)
• minimum depth 5
• disabled while in check
• only triggers when the transposition table provides no move
• reduces depth by exactly one ply

Why conservative

IIR is primarily useful when a deep PV node lacks a TT move to guide move ordering. IronPhoenix already applies LMR to later quiet moves, so this first IIR version keeps the reduction to one ply and avoids stacking broader reductions onto non-PV nodes.

Scope

• only src/search.cpp changes
• existing SEE pruning, LMR, history, PVS, aspiration windows, TT cutoff behavior, comments, and move ordering remain intact

Please SPRT this branch against main by itself before combining it with Null Move Pruning or other search changes.