fakeSeamPair

Build a wired pair of FakeSeams whose FakeSeam.broadcast calls cross-deliver into the other seam's Seam.incoming, matching the send semantics of a real two-peer fabric.

• Each side's peers set contains both hostId and joinerId.

• A FakeSeam.broadcast on one side stamps the sender's FakeSeam.selfId and a monotonically increasing sequence at the receiver.

• The broadcast is also recorded in the sender's FakeSeam.broadcasts list.

• Delivery is synchronous — no coroutine substrate required.

val (host, joiner) = fakeSeamPair(PeerId("host"), PeerId("joiner"))
host.broadcast(byteArrayOf(1, 2, 3))
val frame = joiner.incoming.first()   // Swatch(payload=[1,2,3], sender=PeerId("host"), sequence=1)