SessionExecution vs Location scope

Chose process-global, Session-ID-based SessionExecution with a process-local coordinator, while SessionRunner, model resolution, tools, permissions, and filesystem remain Location-scoped, over making the whole execution stack global or Location-owned. Placement is resolved through SessionStore and LocationServiceMap only when a drain starts, so moved Sessions attach correctly; omitted Location.workspaceID means implicit-local and must not be synthesized.