Z sort override per Shape

Currently, in order to change the render order of a Shape one has to add it to a Group along with additional invisible Shapes in order to nudge the Z sort to the desired position. It would be nice if one could specify a point that, if set, `updateSortValue` would use instead of calculating average Z.

Start by locating the Shape implementation and its updateSortValue method, then trace how render sorting currently uses the average Z value. Done means a Shape can optionally provide a point used for sorting while the existing average-Z behavior remains the default; check the project’s existing tests or examples for the appropriate verification path.

Written by the indexing model from the issue text.