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/_rotterdam/services.nix

@@ -0,0 +1,11 @@
+{
+  services.keyd = {
+    enable = true;
+    keyboards.main = {
+      ids = [ "5653:0001" ];
+      settings.main = {
+        esc = "overload(meta, esc)";
+      };
+    };
+  };
+}