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/hardware-configuration.nix

@@ -0,0 +1,20 @@
+{
+  lib,
+  modulesPath,
+  ...
+}:
+
+{
+  imports = [ (modulesPath + "/profiles/qemu-guest.nix") ];
+
+  boot.initrd.availableKernelModules = [
+    "xhci_pci"
+    "virtio_pci"
+    "virtio_scsi"
+    "usbhid"
+  ];
+
+  networking.useDHCP = lib.mkDefault true;
+
+  nixpkgs.hostPlatform = lib.mkDefault "aarch64-linux";
+}