PolyLabel kept its candidate cells in a sorted List, which costs
O(n^2) through Insert and RemoveAt(0). A 48 x 0.002 sliver searched
to 1e-4 took about 27 s.
Use a PriorityQueue keyed by (MaxDist, insertion order). Equal bounds
still pop newest first, and initial cells in sorted order, so every
existing caller gets bit-identical poles. A frozen copy of the
sorted-list implementation checks this on 1800 random shapes and on
symmetric shapes with tied poles. Swapping the tie order fails 25 of
the differential rows.
A new Find(outer, holes, precision, maxCells) overload stops after
about maxCells distance evaluations and returns the best cell so far.
When the starting grid would exceed a quarter of the budget, it is
coarsened so the whole shape is sampled first. The unbounded overload
behaves as before.
