hex-grid-morphological-close-hull

Compute a unified outer hull around a group of hex tiles that includes multiple disconnected sub-clusters separated by 1-tile gaps. Use when: (1) rendering nested hierarchy hulls (e.g. "package" hull wrapping multiple "sub-region" hex islands), (2) the naive boundary trace outlines each island separately instead of as one bag, (3) flood-fill from outside walks THROUGH the 1-hex gaps and marks them exterior, (4) heuristic "non-adjacent same-group neighbours" over-fills L-shape concavities, (5) axial-only opposite-pair filling misses non-axial bridges between islands. Solution: morphological close (dilate + erode) on the hex grid — fills exactly the interior 1-hex gaps without affecting exterior corners, then trace boundary on the closed shape.