fn start_drag(data: &Drawing<'_>, id: RouteId, pos: Pos2) -> Option<EditRoute>Expand description
Classify where pos lands on route id and return the matching lazy drag
state — reading only, staging no document mutation. The edit plan is built
on the drag’s first Dragging frame (see EditRoute::widget), after the
select→drag tool switch has re-routed the wire.
Priority: waypoint → corner → edge center.