Compile-time normalisation of a via: hop list into a canonical, validated form.

A traversal is an ordered list of hops, each walking one graph edge in one direction. Direction is mechanical and mechanism-independent — it names which end of the stored edge this instance is on:

• :forward — this instance is the edge source; filter source_id = me, follow to target_id.
• :reverse — this instance is the edge target; filter target_id = me, follow to source_id.

Mechanism is :assignment (AssignmentRelationship) or :relationship (both DefinedSimpleRelationship and the general Relationship — a :relationship hop spans either storage, #222). The two axes are independent, so any mechanism × direction combination is a legal hop and chains of any length compose.

User hop forms

• alias (bare atom) — shorthand for {:reverse, assignment: alias} (inherit from your assigner — the common case, and the zero-config default for an omitted via:).

• {:forward | :reverse, assignment: alias}

• {:forward | :reverse, relationship: type} — relationship filtered by type only.

• {:forward | :reverse, relationship: [type: t, alias: a]} — by type and/or alias.

Canonical form

normalize/2 returns {:ok, hops} where each hop is {:forward | :reverse, :assignment | :relationship, selector} — selector %{alias: a} for assignment, %{type: t, alias: a} for relationship (either of t/a may be nil, but not both). On a malformed hop it returns {:error, reason} for the verifier to surface as a DslError.

Used at compile time by TransformInheritedRefs (to inject the calc) and VerifyCharacteristics (to validate). The runtime counterpart is Diffo.Provider.Calculations.Traversal.