Add guarded Strelka-style razoring

[KnightStackSys]

Adds a Strelka-inspired shallow razoring experiment for isolated Elo testing.

Razoring policy

• non-PV nodes only
• skips the root (ply > 0)
• maximum depth 3
• disabled in mate windows
• disabled if any of the four players is in check
• depth 1 uses a +125 cp razor margin
• depths 2-3 use a +300 cp total razor margin
• drops into qsearch to confirm likely fail-low nodes
• preserves Strelka's unusual depth-1 behavior: if the static test says fail-low, the qsearch result is returned even if qsearch itself rises above beta
• at depths 2-3, the qsearch result must still remain below beta before the razor return is accepted

4PC safety guard

IronPhoenix checks Position::inCheck(Color) for every color before allowing razoring. A check against Red, Blue, Yellow, or Green disables the heuristic for that node.

Scope

• only src/search.cpp changes
• existing TT cutoffs, PVS, LMR, SEE pruning, LMP, history, aspiration windows, and evaluation remain otherwise unchanged

This branch is intentionally isolated from IID/IIR and should be SPRT tested directly against main.