pub fn diagonal_route_via(a: TreeCoord, b: TreeCoord) -> Option<TreeCoord>Expand description
For a diagonal edge between a and b, return the “L-bend corner
lot” the renderer should route through — the corner lot that’s
empty. None for non-diagonal pairs (renderer chooses by longer-axis
heuristic instead).
Invariant: if edge_exists(a, b) is true for a diagonal pair, at
least one of the two corner lots is empty (the other diagonal-suppress
rule above guarantees this), so this returns Some for every
existing diagonal edge.