Include holes in ClippedPolygon perimeter

This is documented as measuring only the outermost contours, so changing that would be breaking. I think this may have previously been used for sampling points for mesh sizing, but it seems it's not anymore, so there's nothing stopping us from choosing a more geometrically conventional definition other than the docstring.

Start by locating the ClippedPolygon perimeter implementation and its docstring, then inspect any existing callers or tests that define perimeter behavior. The work is done when hole contours are included consistently, the documented definition matches the behavior, and relevant tests cover the changed result.

Written by the indexing model from the issue text.