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/alexandria.nix

@@ -0,0 +1,42 @@
+{ inputs, ... }:
+{
+  flake.nixosConfigurations.alexandria = inputs.nixpkgs-stable.lib.nixosSystem {
+    system = "x86_64-linux";
+    specialArgs = { inherit inputs; };
+    modules = [
+      inputs.agenix.nixosModules.default
+      { networking.hostName = "alexandria"; }
+      { 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
+
+      # Other aspects based on tags
+      inputs.self.modules.nixos.fwupd
+
+      # Host-specific files (from _alexandria/)
+      ./_alexandria/hardware-configuration.nix
+      ./_alexandria/jellyfin.nix
+      ./_alexandria/kanidm.nix
+      ./_alexandria/nextcloud.nix
+      ./_alexandria/nginx.nix
+      ./_alexandria/unbound.nix
+      ./_alexandria/vaultwarden.nix
+    ];
+  };
+}