Pages with tens of thousands of vector-drawing rects (e.g. architectural
plans with 36k+ rects) caused cluster_rects and sub-rect deduplication to
spend 10-50s in O(n²) loops. No real table has thousands of cell rects,
so once a union-find component exceeds 2000 elements we skip further
comparisons, and skip the O(n²) sub-rect dedup entirely for such pages.
Benchmarks on 8 slow production PDFs show 10-40x speedups on vector-heavy
files (12s→0.3s) with no regression on other file types. All 342 tests
pass.
