Add guarded Internal Iterative Deepening

[KnightStackSys]

Adds a conservative Internal Iterative Deepening (IID) experiment for isolated Elo testing.

IID policy

• applies only at PV nodes
• skips the root (ply > 0)
• minimum depth 7
• disabled while in check
• disabled in mate windows
• only triggers when there is no TT move
• runs a shallower search at depth - 3
• uses the resulting TT move only for move ordering
• preserves the original node depth for the real search

Recursion safety

A thread-local guard prevents IID searches from recursively spawning additional IID searches inside their own subtree.

PV handling

The shallow IID search may populate the PV internally, so the node PV length is reset before the full-depth search resumes.

Scope

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

This branch is intentionally isolated from the IIR branch so IID can be SPRT tested directly against main.