add aspects/hosts/ configurations

Host-specific NixOS configurations for:
- alexandria (server)
- io (desktop)
- rotterdam (desktop)
- trantor (server, aarch64)

Each host has a main config file and _hostname/ directory
with hardware-configuration and other host-specific modules.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

aspects/hosts/_trantor/networking.nix

@@ -0,0 +1,8 @@
+{
+  networking = {
+    firewall = {
+      allowedTCPPorts = [ 25566 ];
+      allowedUDPPorts = [ 25566 ];
+    };
+  };
+}