add a semi-spread example deploy and a check that evaluates it

docs/examples/semi-spread/common.nix

@@ -0,0 +1,21 @@
+# Imported by every machine in the swarm: the swarm's shared facts, the VPN and
+# the names inside it. `services.hyperhive.swarm.*` has to be identical on all
+# of them, so it lives here and nowhere else.
+{
+  imports = [
+    ./wireguard.nix
+    ./dns.nix
+  ];
+
+  services.hyperhive.swarm = {
+    domain = "myswarm.example";
+    # Every hive, including the one a machine does not run. Names only: each
+    # hive's domain defaults to `<name>.myswarm.example`. A hive name may not
+    # contain "hive", "agent" or "swarm", so the machines `hive-one` and
+    # `hive-two` run the hives `one` and `two`.
+    hives = {
+      one = { };
+      two = { };
+    };
+  };
+}