Closes the limitation documented in #1233: an allOf over two distinct recursive components still crashed oasdiff flatten with a stack overflow.

Why this shape resisted #1233

allOf: [$ref NodeA, $ref NodeB], each recursive, merges a child set with two distinct schemas — no lone schema to reuse — so flattenSchemas must run, and its in-flight guard anchors the recursive edge at the result being built. The anchor's value is semantically right: the merged node's recursion is the merged node itself (merge(A,B) recursively equals merge(A,B)). What's wrong is only the representation: the merged node is anonymous, the edge carries no $ref, and a ref-less cycle has no serialized form.

The fix: give the anchor a name

• The merge records every edge the in-flight guard anchors (state.anchored).
• MergeSpec then names each target: a target that is already a named component gets that component's $ref; an anonymous target is hoisted into components.schemas under a generated AllOfMergedN name, and the edge becomes a $ref to it.
• Names are assigned in the document's deterministic walk order — not in the merge's map-iteration order — so identical inputs produce identical output (the Non-deterministic diff --format json output for schemas with multiple reference cycles #1230 lesson applied preemptively).

The fixture now flattens to:

components:
  schemas:
    AllOfMerged1:            # hoisted merged node
      type: object
      properties:
        child:
          $ref: '#/components/schemas/AllOfMerged1'

with the inline occurrence referencing it, and NodeA/NodeB untouched.

Scope note: the naming pass runs in MergeSpec (it needs the components map); the schema-level Merge API is unchanged and can still produce in-memory ref-less cycles for library callers, which the diff's in-flight guard (#1231) handles.

Regression test asserts the hoisted name, the shared identity between the inline node and the component, a successful marshal of the whole document, and byte-identical output across two loads. Full suite and lint pass; breaking --flatten-allof on the shape stays green as before.