dual_decomposition: chord-apex proof + diagrams

- Replace the chord-apex TODO with a full proof by contradiction: assume
  merged != spike, define X, Y, Z, W, lift to G' so that the externals
  inherit \psi(f) = (X, Y, Z, W, W), and split on W in {X, Z}. Either case
  meets the hypothesis of lem:pentagonal-externals, which extends \psi to a
  proper 3-edge-colouring of G' --- contradicting non-3-edge-colourability
  via Tait.
- Add fig:chord-apex-proof: the assumed reduced-dual colouring on top, and
  the two lifted-G' cases (W=Z, W=X) below, rendered on the dodecahedron.
- Add experiments/draw_chord_apex_proof.py.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
This commit is contained in:
2026-05-23 02:31:54 -04:00
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