cas

A keyvalue interface that provides compare-and-swap (optimistic concurrency) for a single key. This is kept separate from `atomics` so a host can grant or deny it independently, and so backends that cannot perform conditional writes can decline just this capability. It is a one-shot, resource-free API: each `swap` carries its own preconditions and reports the current state back on conflict, so the read-modify-write retry loop needs no extra round trips.