DeltaStore.frontier returned the live internal set. A delta built with
parents=store.frontier aliased it, so insert's frontier maintenance
(add + difference_update) emptied the frontier on every insert and
rewrote every stored delta's parents to {} — the CRDT causal chain
degenerated into all-roots. Sopher's bridge now copies on its side
(sopher ADR-0026.1 §2.1); this is the recommended CORE-side defense in
depth: frontier hands out snapshots, and insert copies delta.parents
into the event envelope.
Regression-tested with the exact aliasing pattern (3-delta chain keeps
one head and stable per-delta parents).
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