fn healing_region(reach: impl IntoIterator<Item = GridPoint>) -> RectExpand description
The rectangle to heal: what the wires needing a lattice span, with room around them to get past whatever is refusing them.
The margin is the moat twice over — once because a block shapes the channels that far out, and once more so a detour has somewhere to go. A region too small does not route wrongly, it fails to route, and the leg falls back to an L; that is why the margin is generous and why the unresolved list is what makes this honest.