mockStats is now a thin orchestrator: map each mock through accumulate, then summarize into the report.

Extracted keptIds(order, picks) — a pure helper for the keeper-detection loop.
Extracted accumulate(m, agg, slotCount) — the per-mock fold that was the bulk of the method's nesting/branching.
Named the aggregate shape Agg and reused it, instead of the anonymous inline type.