Disabling a durable observation requires consumer invalidation

Disabling a durable observation requires retiring its readable output, not only stopping its producer. This applies when an observe/shadow process persists receipts, fingerprints, caches, or projections that downstream readers can continue to consume. A safe rollback contract must either gate every reader on current mode or write an explicit inactive/tombstone state; otherwise stale observations remain operationally visible. This does not apply to purely ephemeral observations with no durable or cached consumers.