0365 — Registration is once per instance, and lifecycle sentinels live in pkg/errors

Context. A second Register call could either silently rebind the instance’s lifetime to a new ctx or be refused; and the new lifecycle sentinels were needed by both producer and consumer packages.

Decision. A second Register returns ErrAlreadyRegistered, whether the instance is live (“the first Register’s context still owns this producer’s shutdown” — silent overwrite would change the lifetime’s owner mid-flight) or wound down (“stays down; construct a new MessageProducer”). Revival is a new instance — construction is cheap. Sentinels were hoisted to pkg/errors with component-neutral strings (“not registered”, “shutdown requested”); wrap sites add “producer for topic X” / “consumer group X on topic Y”. Teaching-error templates stay per-package since their snippets name package types.

Consequences. Future heartbeat starts and presence-row writes stay strictly single-shot. Same stdlib-shadowing wart as pkg/context, same moot-at-root answer.