feat(installer): add one-file installer build and custom install ISO

Replace old_installer.py with a curses TUI installer packaged as a
one-file PyInstaller binary (python/installer/build.py, wrapped by
python/installer/package.nix). The default .#installer is patched to run
on foreign Linux live media; the new .#installer-nixos variant keeps its
Nix store interpreter so it runs on NixOS.

Add systems/iso, a minimal NixOS install CD with kernel 6.18 and ZFS 2.4
matching the deployed systems and the installer on PATH; build it with
nix build .#iso. Shared logging and subprocess helpers move out of
common.py into python/logging_config.py and python/process.py.

flake.nix

@@ -65,6 +65,35 @@
 
       devShells = forEachSystem (pkgs: import ./shell.nix { inherit pkgs; });
       formatter = forEachSystem (pkgs: pkgs.treefmt);
+      packages = forEachSystem (
+        pkgs:
+        let
+          installer = pkgs.callPackage ./python/installer/package.nix { };
+          installer-nixos = pkgs.callPackage ./python/installer/package.nix { patchElf = false; };
+        in
+        {
+          inherit installer installer-nixos;
+          default = installer;
+        }
+        // lib.optionalAttrs (pkgs.stdenv.hostPlatform.system == "x86_64-linux") {
+          iso = self.nixosConfigurations.iso.config.system.build.isoImage;
+        }
+      );
+      apps = forEachSystem (
+        pkgs:
+        let
+          system = pkgs.stdenv.hostPlatform.system;
+          installer = {
+            type = "app";
+            program = "${self.packages.${system}.installer}/bin/nixos-installer";
+            meta.description = "One-file NixOS ZFS installer.";
+          };
+        in
+        {
+          inherit installer;
+          default = installer;
+        }
+      );
 
       nixosConfigurations = {
         bob = lib.nixosSystem {
@@ -91,6 +120,12 @@
           ];
           specialArgs = { inherit inputs outputs; };
         };
+        iso = lib.nixosSystem {
+          modules = [
+            ./systems/iso
+          ];
+          specialArgs = { inherit inputs outputs; };
+        };
       };
     };
 }