The System Map subsumes the read-only Graph diagnostic, so remove the page,
its route, and nav entry (the /graph API + useGraph stay — the map and palette
consume them).
Add a host switcher under the sidebar brand: it shows which castle host you're
driving (the is_local node) and, when peers are present, jumps to a peer's
dashboard at castle.<domain>, preserving the current view. Node summaries now
carry gateway_domain so the switcher can build each host's URL.
