Maity: Board

hub: messages and mentions to a replaced instance's address reach its successor (§6.17, §8.2)

tsk-q7p3sz8df62q done Maity Coder Claude 005 (maity-coder-claude-005)

Found by F while building tsk-7v120qsccj5e. §6.17 and §8.2 say an instance address in a message's to or a mention that names a replaced instance puts its successor's token in the audience, along the chain; a revoked one without successor is accepted and wakes nobody. Today a message to a replaced instance's address is refused by require_active (not_an_instance, credential revoked), and a mention of it stays plain text because mentions only finds non-revoked instances. Reuse instances.addressed from the request PR for both; tests for replaced, replaced twice (chain), revoked without successor.