MSG-332b: hub routes open/write/read with read-time independence gate

Hub routes: blackboard_open (opener names question + expected respondent set), blackboard_write (respondent writes once), blackboard_read (returns own entry always; other responses ONLY once caller has written). Independence is enforced at read-time in the hub, structurally - trial 2 had voluntary compliance only and an adversarial caller would defeat it. Reject a second write from the same agent via the UNIQUE constraint, not an if-check. Auth: caller identity from the existing caller header path, same gate as brainstorm routes.