refactor(installer): extract validate_yesno and a NVIDIA driver-floor constant

Four near-identical blocks validated that AUTOLOGIN, NO_GPU_DRIVERS, INSTALL_CLAUDE_CODE, and INSTALL_DEVICE_UDEV_RULES were empty or exactly yes/no. They collapse into a validate_yesno helper the callers guard with `|| exit 1`.

The NVIDIA driver floor of 535 was a magic literal in the comparison and three prose lines. It now lives in a NVIDIA_MIN_DRIVER constant, and the nvidia-preflight test sources that constant alongside the function it extracts.

This is the safe, purely-testable slice of the refactor rollup. The structural extractions that touch the installer's system-mutation paths -- the GPU modalias scan, the stow and retry loops, the display-server/window-manager dispatch, and the snapper/fsck/GRUB/waybar sed blocks -- are split into a follow-up so they get characterization coverage first rather than a blind rewrite.

1 files changed, 57 insertions, 0 deletions

diff --git a/tests/installer-steps/test_validate_yesno.py b/tests/installer-steps/test_validate_yesno.py
new file mode 100644
index 0000000..da78fc5
--- /dev/null
+++ b/tests/installer-steps/test_validate_yesno.py
@@ -0,0 +1,57 @@
+"""Test the validate_yesno config-validation helper.
+
+The installer had four near-identical blocks validating that AUTOLOGIN,
+NO_GPU_DRIVERS, INSTALL_CLAUDE_CODE, and INSTALL_DEVICE_UDEV_RULES are empty or
+exactly yes/no. validate_yesno collapses them into one testable helper: empty
+passes (the default), yes/no pass, anything else fails with a named error.
+
+Method: sed-extract validate_yesno from the real `archsetup` and drive it with
+plain args.
+
+Run from repo root:
+    python3 -m unittest tests.installer-steps.test_validate_yesno
+"""
+
+import os
+import subprocess
+import textwrap
+import unittest
+
+REPO_ROOT = os.path.abspath(os.path.join(os.path.dirname(__file__), "..", ".."))
+ARCHSETUP = os.path.join(REPO_ROOT, "archsetup")
+
+
+def run(name, value):
+    script = textwrap.dedent(f"""\
+        source <(sed -n '/^validate_yesno() {{/,/^}}/p' "{ARCHSETUP}")
+        validate_yesno {name!r} {value!r}
+        echo "RC=$?"
+    """)
+    return subprocess.run(
+        ["bash", "-c", script], capture_output=True, text=True, timeout=10,
+    )
+
+
+class ValidateYesno(unittest.TestCase):
+    def test_yes_passes(self):
+        self.assertIn("RC=0", run("AUTOLOGIN", "yes").stdout)
+
+    def test_no_passes(self):
+        self.assertIn("RC=0", run("AUTOLOGIN", "no").stdout)
+
+    def test_empty_passes(self):
+        self.assertIn("RC=0", run("AUTOLOGIN", "").stdout)
+
+    def test_other_value_fails_with_named_error(self):
+        r = run("NO_GPU_DRIVERS", "maybe")
+        self.assertNotIn("RC=0", r.stdout)
+        self.assertIn("NO_GPU_DRIVERS", r.stderr)
+        self.assertIn("maybe", r.stderr)
+
+    def test_capitalized_yes_fails(self):
+        # The values are compared exactly; "Yes" is not accepted.
+        self.assertNotIn("RC=0", run("AUTOLOGIN", "Yes").stdout)
+
+
+if __name__ == "__main__":
+    unittest.main()