Function kv_cas_path 

pub fn kv_cas_path(namespace: &str, key_b64: &str) -> String

PUT /agdx/kv/{namespace}/{key}/cas: a conditional write (compare-and-swap). The precondition rides the query string (expect_version or expect_absent) and the value rides the raw body, like the plain PUT. A success replies CasCommittedView with the new version, a precondition miss replies 409 with an ErrorBody of code conflict whose detail carries the current version.