Module-level singletons converted to per-session lazy stores

A broad internal refactor replaces numerous module-level mutable globals with a WeakMap-backed lazy holder keyed on the session host, so state is scoped per session rather than shared across the whole process. Affected areas include background-daemon beacon state, the sync/retry engine, background job frames, in-flight monitor state, the plan-file cache and plan slugs, the design tool's plan/approval/grant state, the background-rendezvous socket server, marketplace and checkout state, and OAuth callback tracking maps. Many free functions gained an explicit session/state argument as a result of the change.