Fix msbuild line icon: uniform stroke width for internal division lines

Previously the internal facet division lines appeared double-width because
the line icon was generated by stroking the plain path (which has gaps).
Fixed by stroking each original face path individually and unioning them,
so shared edges merge naturally into single-width lines.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 2 people