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.nix

@@ -0,0 +1,46 @@
+{ inputs, ... }:
+{
+  flake.nixosConfigurations.trantor = inputs.nixpkgs-stable.lib.nixosSystem {
+    system = "aarch64-linux";
+    specialArgs = { inherit inputs; };
+    modules = [
+      inputs.agenix.nixosModules.default
+      { networking.hostName = "trantor"; }
+      { nixpkgs.overlays = [ inputs.agenix.overlays.default inputs.self.overlays.default ]; }
+
+      # Common aspects (always included)
+      inputs.self.modules.nixos.common-boot
+      inputs.self.modules.nixos.common-console
+      inputs.self.modules.nixos.common-firewall
+      inputs.self.modules.nixos.common-locale
+      inputs.self.modules.nixos.common-nix
+      inputs.self.modules.nixos.common-openssh
+      inputs.self.modules.nixos.common-programs
+      inputs.self.modules.nixos.common-security
+      inputs.self.modules.nixos.common-services
+      inputs.self.modules.nixos.common-tailscale
+      inputs.self.modules.nixos.common-users
+
+      # Server aspects
+      inputs.self.modules.nixos.server-boot
+      inputs.self.modules.nixos.server-nix
+      inputs.self.modules.nixos.server-tailscale
+
+      # Factory-generated ephemeral module
+      (inputs.self.factory.ephemeral {
+        rootDevice = "/dev/disk/by-id/scsi-360b207ed25d84372a95d1ecf842f8e20-part2";
+      })
+
+      # Host-specific files (from _trantor/)
+      ./_trantor/hardware-configuration.nix
+      ./_trantor/disko.nix
+      ./_trantor/boot.nix
+      ./_trantor/fail2ban.nix
+      ./_trantor/forgejo.nix
+      ./_trantor/networking.nix
+      ./_trantor/nginx.nix
+      ./_trantor/openssh.nix
+      ./_trantor/unbound.nix
+    ];
+  };
+}