Redraw n=21 witness figures as crossing-free planar graphs

Replace the radial (crossing-heavy) figure with two crossing-free planar
drawings (networkx planar_layout / Chrobak-Payne):
  fig:n21-elgs  -- the six witness Even Level Graphs, parity-coloured, with
                   the bridge-switch-flipped edges dashed red;
  fig:n21-duals -- the six resulting duals, with the introduced bridge edges
                   solid green.
ELG and dual are drawn with independent planar layouts so neither has any
edge crossing (a flip diagonal would otherwise cross other edges when its
quadrilateral is non-convex, which happens for duals 0 and 3). Drop forced
equal aspect so panels fill and labels separate.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

papers/even_level_graph_generators/paper.log

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