nix: give the gateway, resolver and bridge their own enable

`services.hyperhive.gateway.enable`, `gateway.dns.enable` and
`network.enable` replace the `hyperhive.enable` gate on all three. Each
defaults to false; the modules that need one assert it with `mkDefault
true` from inside the guard their own deployment already carries, and
`swarm-required-services.nix` — the module that owns what the
swarm-services toggle implies — asserts all three explicitly.

hive-c0re asserts all three unconditionally, so an ordinary hive keeps
getting them with no opt-in: it is the host's only knowledge that agent
containers exist.

The resolver moves to its own `hive-gateway/dns.nix` so it can be gated
without reindenting the nginx half of the module.

Reinstates `network.enable`, dropping its `mkRemovedOptionModule` shim.
A config still carrying `network.enable = false` from before the removal
now switches the bridge off instead of failing eval.

Also deletes a duplicate `centralToggleOff` fixture in nix/module-eval.nix.
Two sibling slices added it independently (c5f60fd5, ce3b3d94); the merge
was textually clean and left `main` failing to evaluate at all, so this
file could not be gated without removing one.

nix/host-modules/swarm-ui.nix

@@ -132,6 +132,10 @@ in
     # makes the name resolve at all.
     services.hyperhive.gateway.localNames = [ apex ];
 
+    # Gateway only — static files served by nginx, no container of its
+    # own and so nothing to resolve.
+    services.hyperhive.gateway.enable = lib.mkDefault true;
+
     # This UI's own swagger docs, always same-origin (`/api/docs/` below)
     # so — unlike authelia/matrix/forge's entries — this one needs no
     # host name and is never conditional on anything but this module