Test class 3: side-effect emission — verify a downstream entity appears

Tracking issue for Class 3 of the test taxonomy (parent: meta-issue #192). Depends on Class 2 (#189) for shared vocabulary.

Shape

Pre/post state: subject entity unchanged (or transitions per Class 2), but a downstream entity appears in another store as a consequence.
Verifier: search the downstream collection scoped to the subject.

Op examples

Goal

Reuse Class 2's runtimeEntities schema. Add a causedBy cross-reference:

"runtimeEntities": {
  "MessageSubscription": {
    ...,
    "causedBy": [
      { "op": "publishMessage", "linkBy": ["messageName", "correlationKey"] }
    ]
  }
}

The planner emits: act op → witness on the downstream entity scoped by the link fields → assert presence.

Acceptance criteria

1. causedBy field added to runtimeEntities schema.
2. At least one Class 3 generated test (e.g. publishMessage → witness on searchCorrelatedMessageSubscriptions).
3. L3 invariant pinning the chain shape.

Why this falls out of Class 2 cheaply

The witness primitive is the same; only the link-by-fields differ. If Class 2 lands cleanly, this should be a small PR.

Start by tracing the Class 2 runtimeEntities schema and planner, then inspect how publishMessage and searchCorrelatedMessageSubscriptions are represented in generated tests. Add the causedBy relationship and verify that the generated test witnesses the downstream entity using messageName and correlationKey. Confirm the L3 invariant pins the intended chain shape.