broadcastOnRoomReachesOnlyRoomMembers

Core isolation gate: clients A and B on table-7; client C on table-9. A broadcast on table-7 is observed by B and NEVER by C. The assertion on C is structural: C's inbox has zero frames — it was never in the table-7 fanout list. table-9 still delivers its own broadcast to C, proving the sibling room is unaffected.