fakeSeamGroup

Build a wired group of FakeSeams, one per id in peerIds, whose FakeSeam.broadcast calls cross-deliver into every other seam's Seam.incoming — the N-peer generalisation of fakeSeamPair.

• Each seam's peers set contains all of peerIds.

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

• Delivery is synchronous — no coroutine substrate required.