A cut-off line passing exactly through a vertex of the clearance outline
got duplicate intersection hits and fell back to the part's bounding box.
Automatic strip lines sit on a 0.05 lattice that often lands on such
vertices, and the strip search preferred them because the box fallback
measured as less cutting, so planned cut-offs looked box-trimmed while a
slightly moved manual line was trimmed correctly.
Slice the polygonal clearance outline exactly with half-open edges:
crossing vertices count once, touching vertices zero or twice, edges on
the line are excluded whole. Zero clearance and non-polygon outlines keep
the existing path and its conservative fallback.
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