v0.22.0 — an app request can use a second linked item

Another linked item, for one request. Engram links vault items to an app per person, and a role may take several ("many": true), one of them the default. The default is still what fill carries and what the backend's environment holds (ENGRAM_<ROLE>_<FIELD>). A request that names another linked item now arrives with use: the bridge opens it like the fill and hands it to the backend on that one request's line as vault, never in the environment, so the process is not restarted, no other request sees it, and its sealed values are scrubbed from that answer. A use role the call does not fill is refused.

Advertised as app_items in hello. Additive, no protocol bump: a server must not send use to a bridge that did not advertise it. Engram needs 0.22.0 only for this feature; the default item works on 0.21.0. See PROTOCOL.md.