[s28] step 1 — update immutability (ns=test/S28/20260503132141-4391)
[s28]   M1 id=d7e1e60b-f0db-406e-abfb-d405f864730a  initial agent_id=<none>
[s28] step 2 — dedup preserves both agent_ids
[s28]   dedup agents observed: ai:alice,ai:bob
[s28] step 3 — federation sync preserves origin agent_id (settle 8s)
[s28] step 4 — export + import round-trip preserves agent_id