Package-level declarations

The FailureLedger keys an entry by its subscription's identity, its context and name (SubscriptionId), every backend alike: names are unique within a context only, and the contexts of a service whose stores are one store share one ledger. Two subscriptions of one name in two contexts, which park the same aggregate in the same partition, never see, list, count, re-park, amend or resolve each other's entries, through any method of the port.

The FailureLedger contract (documentation/designs/04-subscriptions.md §2) every backend passes. A backend's test extends it and supplies a ledger from createLedger.