skills: no step probes what names a position whose node kind a change alters #1696

Problem

A lowering that changes the kind of node a position lowers to, such as a Model becoming a Union, changes it for everything that names the position: a discriminator mapping or an implicit route to it, an allOf composing it as a Base, a $ref from another position. No step in morphic-reviewing-prs' angles, and no lesson in morphic-openapi-compiler, asks a review to compile the changed spelling as each such consumer names it. references/robustness.md#new-route-open-bugs rewrites the open repros of the old route, which finds nothing when the old route's failure was never filed.

The case that showed it: #1688 distributed a union written on an allOf branch, and a union with inline branches beside a body, into a Union of composed models. Below a discriminated base whose mapping routes to the schema, pass.Validate refused the mapping. The same failure already stood on main for $ref branches beside the allOf, unfiled, so the PR's searches found nothing and its review rounds passed it.

openapi: 3.1.0
info: {title: t, version: "1"}
paths: {}
components:
  schemas:
    Pet:
      type: object
      properties: {kind: {type: string}}
      discriminator: {propertyName: kind, mapping: {dog: '#/components/schemas/Dog'}}
    Small: {type: object}
    Big: {type: object}
    Dog:
      allOf:
        - $ref: '#/components/schemas/Pet'
        - oneOf: [{$ref: '#/components/schemas/Small'}, {$ref: '#/components/schemas/Big'}]

morphic validate --verify dog.yaml
main @0610ee63f: exit 0
#1688 @5dea18eee: exit 1, error pass/discriminator-missing-variant t/openapi/components/schemas/Pet: discriminator mapping "dog" on t/openapi/components/schemas/Pet references "t/openapi/components/schemas/Dog", which is not a variant of it

Direction

A draft: a Correctness or Contract step. For each position whose node kind the change alters, compile it as each consumer the IR has for the old kind names it, a mapping and an implicit route, an allOf composing it as Base, a $ref, an encoding key, with --verify, on main and the head, and read every diagnostic.

Related: #1688, #1694

Priority: P4, rule 1 (housekeeping: a skills rule changes no lowering, validation, IR or emit-contract behaviour). The probe above (morphic @5dea18eee and @0610ee63f).