`transport::watcher` was published as `pub mod`, which makes `LinkWatcher` and
its group constants part of the crate's compatibility surface. Every caller is
in-crate — `node::netmon` and its tests, reached as `crate::transport::watcher`
— so nothing outside the crate is relying on it, and publishing it would commit
the library to the shape before anyone has asked for that shape.
Narrowed to `pub(crate) mod`. This is a one-line change now and a breaking one
after a release carries it.