Shrink the Repro

Given a boolean test over a bitmask (bit i = keep element i), implement unsigned int shrink(unsigned int failing, size_t width, pred_fn pred) — pred(bits) returns true iff the masked element subset still fails; return the smallest subset (as bits) that still fails. Simple greedy: try removing each kept element from the highest index down; keep the removal if it still fails. Never reduce below a single element — a repro must still exercise the failure.