A one-shot record of post-mortem artifacts we have already emitted a capsule for.

What it is

A public ETS set keyed by the artifact's sha256:... id. mark_seen/1 returns true if the id was new (so a caller should emit), false if it had already been recorded. That is atomic in one ETS op — :ets.insert_new/2 — so two concurrent sweeps on the same dump cannot both emit.

Not persistent across BEAM restarts. That is deliberate: a fresh BEAM should re-emit the dumps it finds on disk, because the recipient of those capsules (the defect bus + subscribers) is also fresh. Persisting the seen-set would silence the exact restart-and-look-again pattern that recovers a defect an earlier BEAM's bus never got to observe.

Same pattern as Mob.Agent.Receipts.Owner and Mob.Invariant.Owner: the table is :public and named, its lifecycle owner is unlinked, and the write path never goes through a GenServer mailbox.