Status: IN
Both messaging and stream processing use one-directional state progressions — read cursors that only advance, windows that never reopen — to eliminate entire classes of regression bugs where state could move backwards.
chat read cursors are monotonic (never re-mark as unread), aggregation windows follow OPEN→CLOSED→FINALIZED with no reversal — both are state ratchets that make backwards movement structurally impossible
Depends on (SL): chat-monotonic-read-progress, watermark-finalization-is-irreversible