Add missing cid to PublicationForm test fixture

The Decision 0013 merge made `cid` a required field on the `Publication`
type (captured for the post's associatedRefs strongRef), but the
save-lifecycle test's `existing` fixture was not updated, so `npm run
check` was already failing on a clean trunk with TS2741.

Set a placeholder `cid` on the fixture to restore a green typecheck.
This is the literal value the test never inspects — it only needs the
shape to satisfy `Publication`.