Read mesh host map from clan zerotier vars instead of hardcoding

The control/ns1/ns2 mesh IPs and the /88 subnet were duplicated literals in
mesh-hosts.nix. clan-core's zerotier generator already writes each machine's IP
as a public var (vars/per-machine/<m>/zerotier/zerotier-ip), so read from there
and derive the subnet from zerotier-network-id. Pure refactor: the rendered
values are identical and the system derivation hash is unchanged.

modules/monitoring/server.nix

@@ -9,7 +9,7 @@
   ...
 }:
 let
-  mesh = import ../mesh-hosts.nix;
+  mesh = import ../mesh-hosts.nix { inherit config lib; };
   vmPort = 8428;
   grafanaPort = 3000;
   controlV6 = mesh.hosts.control;