bufferCleared

Reports a local clear after the active screen and history have been erased. Called synchronously under session mutation serialization, before later output. Invalidate producer-owned anchors for buffer; preserve metadata for the other buffer. Old line identities cannot resolve after this callback. Host models keep ownership of their text, metadata, and notifications. Do not block on UI work, mutate the session, or close it. Exceptions propagate to the clear caller after the grid has changed.