openapi: an outside $ref weakens the name of an inline position under /paths

[OmarAlJarrah]

Summary

#353 closed the order-dependent hint at the four inline structural positions —
items, additionalProperties, a patternProperties entry, a prefixItems slot —
by having the pointer walk (structuralPointerHint) replay what the structural
lowering composes. That derivation is confined to /components/schemas/…, and the
remainder is this issue.

Under /paths the enclosing hint comes from an operationId, a response, or a
media-type key, none of which the pointer records. With no reference to it, the
position is named by the structural lowering:

t/anon/paths/~1x/get/responses/200/content/application~1json/schema/items
hint=response_item

Add a component pointing at that same pointer, and the node is named items
instead — the last segment of the pointer, which is all the walk has there.

It is not an order dependence

Worth stating precisely, because #353 was. Components lower before paths, so a
reference from components always interns the node first, in either spelling of
the document. Both orders produce items, and the document stays deterministic.

What is left is that a node's name depends on whether some unrelated schema
elsewhere points at it. A/items in components is A_item whether or not anything
references it; the same position under /paths is response_item or items
depending on that. It is a weaker defect than #353's and a real one: the name an
emitter renders should be a property of the declaration, not of the document's
reference graph.

What closing it would take

1. Make the reference side never name the node. A node's hint would come only
   from the lowering that reaches it through its enclosing declaration; a lowering
   arriving through an outside $ref would intern the node but not claim its name,
   and the declaration would set it on arrival. That matches the IR's own principle —
   a name comes from a declaration, not from a reference to it — but it means the
   type registry has to distinguish an authoritative namer from a provisional one,
   which is new machinery in compilers/compile rather than a change to one hint
   function.
2. Decide the weaker spelling names the node at these positions everywhere.
   Cheaper, and worse: the enclosing context is most of what makes an anonymous
   node's hint usable to an emitter.

Direction 1 looks right and is the larger change. It would also subsume
branchPointerHint and structuralPointerHint, which exist only to make two namers
agree — with direction 1 there is one namer, and the agreement is structural rather
than maintained.

Where it is pinned

TestInlinePosition_UnderPathsTakesTheWeakerName
(compilers/openapi/internal/schema/schema_test.go) pins all three values — the
unreferenced response_item and the referenced items in both declaration orders —
and names this issue.

Note that orderInvariantIR() no longer excludes ir.Naming.Hint at all: #353 made
the field comparable, and this remainder is not an order dependence, so nothing here
needs hiding from that comparison.