v0.21.3

Added

• Session.Disconnected — closes when the session's transport ends, before
  in-flight handlers necessarily return; Done keeps its settle-after-handlers
  meaning, and Disconnected never closes after Done. The internal wire layer
  already told the two edges apart, but the root Session exposed only Done,
  which fires after every in-flight handler returns — so fusekit's
  native-session supervision had no way to present a FUSE mount as unavailable
  the moment its backing process died mid-handler; the loss only surfaced after
  blocked reads drained. A spawned session closes Disconnected the same way,
  when its handoff channel ends.