feat: introduce guests as nested-container definitions

Add a third auto-loaded kind beside the host and the module: the guest, a
reusable definition under guests/ that a host enables like a module and that
realizes its interior as a systemd-nspawn nested container.

Split the shared base config so a guest can stand on it.
base.nix now holds the substrate both bases share — the primary user,
home-manager, and the unstable/stable overlays.
system.nix keeps the host-only machinery, and a new guest.nix is the slim
guest-base: it imports the full modules tree, pins the interior release, and
auto-enables the toolkit bundle and SSH so any guest is workable on sight.

Give modules.ssh a guest flavor.
A host restores its host keys from secrets as before, while a guest sets
hostKeys.restore = false, names no sops files, and self-generates a host key,
so it holds no age key of its own.

The lib grows a guest helper that declares the guests.<path> namespace with an
enable and a backend field.
Only the container backend is built; microvm is a reserved value that trips a
clear build-time assertion rather than silently building nothing.

A sample guest exercises the whole path, and neogaia enables it, so the guest
interior builds through the existing nix flake check seam.

base.nix

@@ -0,0 +1,82 @@
+{
+  config,
+  lib,
+  inputs,
+  ...
+}:
+# The shared foundation both the host base and the guest-base build on: the
+# primary user, home-manager, and the fresher/pinned package overlays.
+let
+  inherit (lib) mkOption types;
+  user = config.user;
+
+  # Args to instantiate an extra nixpkgs source on the base platform.
+  pinArgs = prev: {
+    inherit (prev.stdenv.hostPlatform) system;
+    config.allowUnfree = true;
+  };
+in
+{
+  imports = [ inputs.home-manager.nixosModules.home-manager ];
+
+  options.user = {
+    name = mkOption {
+      type = types.str;
+      default = "alexion";
+      description = ''
+        The primary interactive user this system is built for. Drives both the
+        system account and the home-manager user in lockstep.
+      '';
+    };
+    description = mkOption {
+      type = types.str;
+      default = "Alexion";
+      description = "Human-readable description (GECOS field) for the primary user.";
+    };
+  };
+
+  config = {
+    # Reach fresher packages with `unstable.<name>` or pin with `stable.<name>`.
+    nixpkgs.overlays = [
+      (_final: prev: {
+        unstable = import inputs.nixpkgs-unstable (pinArgs prev);
+        stable = import inputs.nixpkgs-stable (pinArgs prev);
+      })
+    ];
+    nixpkgs.config.allowUnfree = true;
+
+    # Flakes, so `nixos-rebuild switch` works from the console and a direnv
+    # `use flake` resolves inside a guest.
+    nix.settings.experimental-features = [
+      "nix-command"
+      "flakes"
+    ];
+
+    # Primary user.
+    # The wheel group is the way in, since root is locked.
+    # No password is set here, since that is host-only.
+    # A guest therefore has none and is reached by SSH key or `machinectl`.
+    users.users.${user.name} = {
+      isNormalUser = true;
+      description = user.description;
+      extraGroups = [ "wheel" ];
+    };
+
+    # home-manager as a NixOS module: one build produces the system and user
+    # environment together, sharing the system's pkgs and installing user
+    # packages into the system profile.
+    home-manager = {
+      useGlobalPkgs = true;
+      useUserPackages = true;
+      extraSpecialArgs = {
+        inherit inputs;
+        my = inputs.self.lib;
+      };
+      users.${user.name} = {
+        home.username = user.name;
+        home.homeDirectory = "/home/${user.name}";
+        home.stateVersion = "26.05";
+      };
+    };
+  };
+}