The immediate registration attempt runs after the subscription is
installed; a synchronous failure (declared slot already occupied) escaped
the constructor without returning a handle, leaving a subscription owned by
nothing — later ledger flushes would fire it against a failed fiber.
Construction failure now unsubscribes before rethrowing, with a dedicated
deferred.spec covering both arms (ds-review-bot on the flow registrations;
the effect-level rollback in the flow packages keeps covering earlier
successfully-constructed deferrals).
creatixchu
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