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).
The five settings-surface registrants carried near-identical
spec-check/ledger-judge/subscribe scaffolding (three jscpd clones);
ui-slots now owns deferRegistration() — ledger-judged presence, refresh
for registrant-localized labels, one-call disposal — and every
registrant shrinks to its registration body.
creatixchu