The construction-rollback + late-conflict-rollback + loud-rethrow block was
about to be a verbatim clone across the two flow packages; ui-slots now owns
it as deferGroupRegistration (one occupant, several holes, as a unit), with
direct specs for all three arms, and the native flow consumes it.
Holes declared after two flow providers activated left the loser throwing
out of the slot flush with partial occupancy. deferRegistration gains an
onFailure channel (late failures unsubscribe, then hand over instead of
throwing through the flush); the native flow's pair rolls back wholesale
and re-raises on the global channel the boot's fail-loud handler owns.
Duplicate rows of the SAME package stay silently idempotent (the component
identity guard skips an occupied hole).
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).
