A persistence API that a document database can implement

The one unchecked item on the 4.0 roadmap (#988): "Refactor persistence API to be more document database friendly" (#814 was the original ask). 4.0 reshaped `IJobStore` (queries, batched acquisition, fire-instance listings, context objects, default interface members) and pins the contract with a shared contract suite, which is most of the groundwork; what remains is a design for a store without SQL semantics (no row locks, no `SELECT … FOR UPDATE`, conditional writes instead), an in-repo reference implementation or a documented one, and whatever the contract suite needs to admit it. 4.x, additive.