Coordinate overlapping refreshes

Two calls can start in order and finish in the opposite order. Select policy from the domain requirement:

• serialize when every operation must observe the preceding result;
• compare a revision before commit when stale responses must be discarded;
• cancel obsolete I/O when the transport supports cancellation;
• accept last-completion-wins only when completion time is the intended authority.

Ark Data Layer deliberately does not impose one concurrency policy on every Repository. If the operation originates in UseCase Forge, its execution instructions can own admission, restart, and grouping before the Repository method is invoked.