zbus 4.4 does not expose a glib feature; my earlier draft was wrong about
that. Switch the plan to use the workspace tokio config and install a
multi-thread tokio runtime in main.rs, with the runtime guard kept alive for
the lifetime of app.run(). glib::MainContext::spawn_local still drives the
widget-touching closures; tokio's reactor wakes them.
Also: hoist futures-util into the Task 1 dep bundle so Task 6 doesn't need
a separate Cargo.toml edit.
a17f70c4ac