memberInboxCapacity

How many unread frames the rooms under test hold per member for Room.incomingFrom — the depth the boundary and overflow obligations (4c)–(4e) measure against.

Defaulted to the reference's own value because the reference is what this suite builds. A harness over a room with a different depth overrides it; leaving it at the default would put those obligations one boundary away from the room's real one, where they measure nothing.