roomInputInView function
h's input through the view adapter (spec 10 D4): a wall's neighbours
are view.neighbours(h) carrying WallParams, ascending. Memoised per
view and handle. Null for a handle that is not a live wall or
separator of the view, and for a degenerate one.
Implementation
RoomInput? roomInputInView(ParametricView view, Handle h) {
final memo = _inputsByView[view] ??= <Handle, RoomInput?>{};
if (memo.containsKey(h)) return memo[h];
return memo[h] = _inputInView(view, h);
}