placeSourceInView function
What a room traces among inside generate and diagnose (spec 10 D7):
the view's placedIn, roomInputInView, and U the union of
placeBoxOf(h) over placedIn of the whole plane (Ruling 10-13), the
engine having already turned a non-finite box into null. Memoised per
view, U included.
Implementation
PlaceSource placeSourceInView(ParametricView view) =>
_sourceByView[view] ??= _ViewSource(view);