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/process.py

@@ -0,0 +1,52 @@
+"""Small subprocess helpers."""
+
+from __future__ import annotations
+
+import logging
+import shutil
+from subprocess import run
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from collections.abc import Iterable, Sequence
+
+logger = logging.getLogger(__name__)
+
+
+class CommandError(RuntimeError):
+    """Raised when an external command fails."""
+
+    def __init__(self, command: Sequence[str], returncode: int, stderr: str) -> None:
+        """Store command failure details."""
+        command_text = " ".join(command)
+        super().__init__(f"Failed to run command {command_text!r}: exit {returncode}\n{stderr}")
+        self.command = command
+        self.returncode = returncode
+        self.stderr = stderr
+
+
+class MissingCommandsError(RuntimeError):
+    """Raised when required external commands are not available."""
+
+    def __init__(self, commands: Sequence[str]) -> None:
+        """Store missing command details."""
+        missing = ", ".join(commands)
+        super().__init__(f"Missing required installer commands: {missing}")
+        self.commands = commands
+
+
+def require_commands(commands: Iterable[str]) -> None:
+    """Raise when one or more executables are missing from PATH."""
+    missing_commands = sorted({command for command in commands if shutil.which(command) is None})
+    if missing_commands:
+        raise MissingCommandsError(missing_commands)
+
+
+def run_output(command: Sequence[str]) -> str:
+    """Run a command and return stdout."""
+    logger.debug("running command=%s", command)
+    result = run(command, capture_output=True, text=True, check=False)
+    if result.returncode != 0:
+        raise CommandError(command, result.returncode, result.stderr)
+
+    return result.stdout