Why: an unset object store must keep artifact bytes available to the other process, which the store-level test does not execute.
Evidence: cross_artifact_roundtrip failed with EOF on backend c6fb130 and passed on 46073d30 (SELF, isolated u7drill-k4).
Impact: Dev now builds the fixed backend. Second-instance write traffic stays closed.
Refs: modules/backend tests/integration/multi-instance/multi_instance_test.go
Co-authored-by: Cursor <cursoragent@cursor.com>