Use AsyncLocal for closed reference logical-context slots

Opt-in in-process port of a static reference property backed only by a private constant CallContext slot, with an as-cast getter and a direct value setter. Store the object reference in a private AsyncLocal<object> initialized first, retaining the original as-cast and value symbols. Require one key literal, its declaration/two bound references and no other context operations/type references. Exclude partial/generic/proxy owners, key escapes, special slots, unsupported imports/comments and conditional files. External named-slot/remoting/trust contracts require separate review. Not composed into version upgrades.