inMemoryMeshOfSize

Build a fully-connected in-memory mesh of n peers using connectionPair links.

For each unordered pair (i, j) one connectionPair is created. Peer i gets the first end, peer j gets the second. All meshSeam calls run concurrently so the Hello preambles cross in parallel (required — serial would deadlock).

Returns one Seam per peer in index order. All inter-peer handshakes are complete before this function returns.