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.

python/logging_config.py

@@ -0,0 +1,16 @@
+"""Logging helpers shared by command-line tools."""
+
+from __future__ import annotations
+
+import logging
+import sys
+
+
+def configure_logger(level: str = "INFO") -> None:
+    """Configure process-wide logging."""
+    logging.basicConfig(
+        level=level,
+        datefmt="%Y-%m-%dT%H:%M:%S%z",
+        format="%(asctime)s %(levelname)s %(filename)s:%(lineno)d - %(message)s",
+        handlers=[logging.StreamHandler(sys.stdout)],
+    )