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.

guest.nix

@@ -0,0 +1,31 @@
+{
+  my,
+  inputs,
+  lib,
+  ...
+}:
+# The guest-base: the slim foundation every nested guest's interior stands on.
+# It imports the full modules tree so any module is available to enable inside a
+# guest, and stands on the same shared base a host does.
+{
+  imports = my.collectNixFiles (inputs.self + "/modules") ++ [
+    (inputs.self + "/base.nix")
+
+    # The modules tree reaches for these option namespaces, so they must be
+    # declared for the tree to evaluate even where a guest leaves them off.
+    inputs.sops-nix.nixosModules.sops
+    inputs.stylix.nixosModules.stylix
+  ];
+
+  # A nested container has no per-host `default.nix` to pin its release.
+  system.stateVersion = "26.05";
+
+  # The baseline toolset and SSH access, so any guest shelled into is a workable
+  # environment without per-guest wiring.
+  modules.toolkit.enable = lib.mkDefault true;
+  modules.ssh.enable = lib.mkDefault true;
+
+  # A guest carries no host identity, so it presents a self-generated host key
+  # rather than restoring one from secrets.
+  modules.ssh.hostKeys.restore = lib.mkDefault false;
+}