Backing calculation for inherited_party DSL declarations.

Walks the instance graph along the via: hop chain (the shared Diffo.Provider.Calculations.Traversal — :forward/:reverse over assignment / relationship edges), then reads each reached instance's PartyRef records at the declared source_role, optionally collapsing the result to one end. Injected automatically by TransformInheritedRefs — do not reference this module directly; use the inherited_party DSL entity instead.

via: reaches the instance that holds the ref; the source_role deref is a fixed terminal step, not a via: hop. Routing through a party — the ref graph — is a calculation concern, not a via: capability (see #227).

See Diffo.Provider.Extension.InheritedPartyDeclaration for the DSL options.

Result shape

Without collapse, a list per input record — one or more Diffo.Provider.Party values per reached instance that carries a PartyRef at source_role, or %Diffo.Unknown{} when an instance is reached but has no PartyRef there. With collapse: :first | :last, a single value (or nil). When no instance is reached at all, [] (or nil when collapsing).

Reason vocabulary

PartyRef is a universal indirection (no cross-world dispatch), so only one reason:

• :role_not_declared — instance reached but its PartyRef records carry no entry at source_role. :context is %{source_id: id, role: source_role}.

:world stamping

TransformInheritedRefs passes the consumer's resource as :world at compile time; each emitted %Diffo.Unknown{} stamps it.