Where two boundaries of a projected face family touch at a single vertex, an even number of boundary edges meet there. The walk now stays on the face it came along (or takes the first edge in geometric order), so every loop closes; an odd count is still refused as a real branch. Adds a test with two squares meeting at a corner.
Projecting a cylinder's wall or the face of a round along a circular edge
failed with "face projection has an open or invalid boundary": these
surfaces are stored in halves joined by hidden seams, and the projection
dropped the seams but kept only the picked half. It now projects the whole
patch, the face as drawn, through the same surface_patch the viewport uses
to highlight it.
Mqrius