When writing a hatched region with holes to DXF/DWG, every NaN-separated ring was created with BoundaryPath::external(), so consumers treated inner loops as outer islands instead of holes. Only the first (outer) ring should carry the external / outermost flags; hole rings are left unflagged. A picked big-minus-small hatch now persists with the small rectangle as a real hole.
Adds a regression test asserting only the first boundary path is flagged external.
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