FilledByConstruction

The joiner's roster holds a remote by construction, so it would hold one whether or not join() recorded anything. Two in-tree shapes, worth distinguishing because they have different fixes:

• a shared roster — both ends read one registry object, so registering the second seam fills both (InMemoryLoom, ControllableLoom, and GossipSeam, whose peers delegates to its base). Splitting the fixture means giving each end its own backend.

• a seeded roster — the joiner's seam is constructed with the remote's id, so its roster is a constructor literal (LinkSeam's setOf(selfId, remoteId), reached by every identified(…) harness). Splitting the fixture is not enough here: the seam would have to learn the id rather than be handed it.