Reusable Matrix user/address reachability helper

Reusable Matrix user/address reachability helper: a task in MiMo-V2.6-RL-harbor-code: MiMo-V2.6-RL Code (Harbor) (Harbor dataset). Our Matrix transport currently buries all of its "is this peer node online?" bookkeeping inside the transport object. Other services (e.g. the pathfinding service) now…