SessionExecution late placement

Chose process-global, Session-ID-based SessionExecution with placement resolved only when draining over Session-owned layers because execution must discover current placement through SessionStore and LocationServiceMap. No layer takes a Session ID; Location-scoped dependencies remain governed by [[01a07ea4-921d-7086-a61c-3f08a8705e46]]. Idle or locally unowned interruption is a no-op, while the public API rejects unknown Sessions.