New paper introducing the dual (inner/weak dual) of a maximal planar graph,
dual depth (BFS-derived min level over a face's vertices), and a Tait-based
framing of a minimal 4CT counterexample via nested level duals. Includes a
dual-depth figure and its generator. Shelved per closing note in favour of an
alternative approach.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
