Centralize shared values into variables.nix

One file (variables.nix) holding every value that was previously
hardcoded and repeated across modules: LAN domain/CIDR, home/tailnet
domains, cross-host references (nix-cache substituter hostname, NFS
server hostname, remote-builder user), PXE/PBS IPs, timezone, and the
primary username.

Wired in via flake.nix's specialArgs (and home-manager's
extraSpecialArgs for the two home.nix files), so any module picks it
up by just adding `vars` to its function arguments — no explicit
import needed. Two hosts (nix-cache, server) now derive their own
networking.hostName from the same variable other hosts use to reach
them, so there's exactly one place to change either identifier.

Purely mechanical: every substituted value matches what was already
there, confirmed by identical toplevel .drv paths for all 17 targets
before and after.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

flake.nix

@@ -23,6 +23,7 @@
     let
       system = "x86_64-linux";
       inherit (nixpkgs) lib;
+      vars = import ./variables.nix;
 
       # Generates a nixosConfiguration from a platform (what it runs on) and
       # a build type (what it's for), plus the per-identity host.nix that
@@ -46,6 +47,7 @@
               home-manager = {
                 useGlobalPkgs = true;
                 useUserPackages = true;
+                extraSpecialArgs = { inherit vars; };
                 users.nixos = import homeFile;
               };
             }
@@ -53,7 +55,7 @@
             ./modules/nix-cache/client.nix
             ./modules/remote-builder-client.nix
           ];
-          specialArgs = { inherit inputs; };
+          specialArgs = { inherit inputs vars; };
         };
 
       # Generated platform x build-type matrix. pxe-boot has no linode