R6: specialization equivalences (AHK umbrella #103)

[cameronfreer]

Child of #103; depends on R1–R3 (bridges land incrementally as those complete). Show that the concrete exchangeability campaigns are specializations of the generic relational framework — via explicit transport equivalences, not necessarily definitional instances.

Specialization map (corrected):

• directed graphs (Umbrella: Directed Diaconis–Janson and digraphons (DJ §9, Cai–Ackerman–Freer) #84 / D1: directed graph foundations (InfiniteDigraph space) #85 / D2: finite and infinite exchangeable digraph laws #86) — one sort, one binary relation, ordered arguments, diagonal permitted; Mathlib Digraph relates to the Boolean relational carrier by an explicit measurable / homeomorphic equivalence (not a definitional instance);
• simple graphs (Graphon-valued de Finetti: the Diaconis–Janson graphon-mixture correspondence #33 campaign) — a measurable invariant subspace / subtype: the symmetric, irreflexive sub-structures of the one-sort binary signature;
• tournaments — the completeness + antisymmetry invariant subspace of the digraph signature;
• hypergraphs — the symmetry + irreflexivity invariant subspace at higher arity;
• bipartite / separately exchangeable — two sorts, one cross-sort binary relation (a future issue).

Stage the bridge explicitly, matching the R-layers:

• R1 bridge: carrier / measurable-space (and topological) equivalences;
• R2 bridge: law and consistency equivalences;
• R3 bridge: dissociation / ergodicity transport;
• R4 / R5 bridge: functional-representation transport.

Bridge PRs may be incremental — the R1 / R2 digraph bridge can land right after R1 / R2 without waiting for R4 / R5.