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/hive-matrix.nix

@@ -705,6 +705,11 @@ in
     # than fronted, so there is no name to claim and no vhost to serve —
     # every clause below carries that guard.
     services.hyperhive.gateway.localNames = lib.optional (cfg.gatewayHost != null) cfg.gatewayHost;
+    services.hyperhive.gateway.enable = lib.mkIf (cfg.gatewayHost != null) (lib.mkDefault true);
+
+    # Unguarded, unlike the two above: the homeserver container resolves
+    # through the hive's dnsmasq whether or not the gateway fronts it.
+    services.hyperhive.gateway.dns.enable = lib.mkDefault true;
 
     # The homeserver's own journal (`tuwunel` is the unit name inside the
     # container, whatever the nixpkgs option is called), plus the host-side