5552e07803
heawood_labelling(): depth-seeded force-first +/-1 labelling of the source-dual cut, targeting sum ≡ 0 (mod 3) on each dual face (vertex link), with bookkeeping for seeds, forced fills, unforceable faces, and failing faces. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>