Constant ROOM_WATERMARK_TABLE 

pub const ROOM_WATERMARK_TABLE: &'static str;

The §4.2 downgrade fence: (doc, flush_seq), co-committed monotonically in every room flush’s transaction (ClusterConsumer::commit_room_flush). Cross-authority cvs are incomparable, so the fence is data that RIDES THE ECHO: a downgraded client keeps its frozen ghost source until its daemon subscription delivers flush_seq ≥ finalFlushSeq — proof the store it fell back to holds the room’s final flush, whatever the authority shape (single daemon / lagging read-follower / PG).