feat(Archive): three definitions of Langer graph are equivalent via Sabidussi

The Langer graph has three independent definitions:
1. Algebraic — via the Zorn product on Q(6,2)
2. Geometric — distance-2 in the Tutte 12-cage restricted to points
3. Group-theoretic — Sab(G₂(2), H₁₉₂, D)

The equivalence (1) ≃g (3) is langerSabidussiIso (Sabidussi representation).
The equivalence (2) ≃g (3) is dist2SabidussiIso (same connection set D at
vertex 0, proved via graphs_agree_at_zero). Composing gives (1) ≃g (2):
langer_iso_tutte12_distance2.

The key: both graphs map to the SAME coset graph because they share the
same connection set at vertex 0 (connectionSet_eq). The orbit-stabilizer
bijection sabidussiEquiv depends only on the group action, not the graph.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: RaggedR