users.displayName NOT NULL DEFAULT '' allows silent empty-name insert (siblings CHECK-reject)

mars users.displayName is NOT NULL DEFAULT '' — an insert with no name supplied succeeds silently with an empty string instead of being rejected. Terra/enamel use NOT NULL + btrim(x)<>'' CHECK, which rejects the same insert outright. Same stated intent (name required), opposite behavior — mars fails silently where siblings fail loud. Same shape pluto already remediated elsewhere per terra. Found incidentally by db-mars-cc during the terra lorena.salomone provisioning lookup, 2026-07-30. Fix: add a CHECK constraint matching terra/enamel/pluto's shape.