Extends Definition 1.2 (level edge) to also define "interlevel edge"
for the primal, and adds Definition 1.3 (level/interlevel dual edge)
classifying dual edges by whether they cross a level or interlevel
primal edge.
Useful downstream: in coloring_nested_tire_graphs, the partial tire
dual's edges can now be classified cleanly as level or interlevel
dual edges using the same vocabulary, instead of ad hoc "interior
annular edge" / "spoke edge" naming.
Paper stays at 4 pages.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
didericis
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