Add file integrity monitoring (Tripwire-style), auto-refreshed via pacman hook

Detects binary/config tampering by content hash — catching a malicious swap
even when mtime is preserved (the gap in mtime-based persistence checks). Two
engines split by file ownership:

- fim.py hash baseline: SHA-256 (+ mode/uid/gid/size) of security-critical
  files the package manager doesn't track (/usr/local, /etc configs, systemd
  units, SSH keys). The baseline refreshes ONLY via fim-update / the pacman
  hook, so a flagged change stays flagged until acknowledged (Tripwire
  semantics). Alerts: fim_modified 100017 / fim_added 100018 / fim_removed 100019.
- package verification: `pacman -Qkk` checks package-owned binaries against the
  distro's own signed checksums — no baseline to maintain, implicitly current
  because the package DB updates on every upgrade. fim_pkg_modified 100020.

Auto-update on system updates: a pacman PostTransaction hook runs
`enodia-sentinel fim-update` after every install/upgrade/remove, so legitimate
package changes never alert — no manual `tripwire --update`.

- daemon: backgrounded FIM scan + optional pkg-verify on slow cadences, feeding
  the normal alert/snapshot/push pipeline; baseline loaded/built at startup
- cli: fim-baseline / fim-update / fim-check [--packages]
- config: fim_enabled, fim_paths, fim_scan_interval, fim_pkg_verify[_interval]
- packaging: ship + install the pacman hook (Makefile + PKGBUILD)
- tests: +7 (hashing, diff incl. mtime-preserving tamper, pacman -Qkk parse).
  72/72 pass. Verified end-to-end: a content swap with preserved mtime is caught.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_fim.py

@@ -0,0 +1,102 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+"""Tests for file integrity monitoring: hashing, diff, alerts, pacman parse."""
+import os
+import tempfile
+import unittest
+from pathlib import Path
+
+from enodia_sentinel import fim
+from enodia_sentinel.alert import Severity
+
+
+class TestHashAndScan(unittest.TestCase):
+    def setUp(self):
+        self.d = tempfile.TemporaryDirectory()
+        self.tmp = Path(self.d.name)
+
+    def tearDown(self):
+        self.d.cleanup()
+
+    def test_hash_stable_and_sensitive(self):
+        p = self.tmp / "f"
+        p.write_text("hello")
+        h1 = fim.hash_file(str(p))
+        self.assertEqual(h1, fim.hash_file(str(p)))
+        p.write_text("hell0")
+        self.assertNotEqual(h1, fim.hash_file(str(p)))
+
+    def test_scan_collects_entries(self):
+        (self.tmp / "a").write_text("x")
+        (self.tmp / "b").write_text("y")
+        scan = fim.scan_paths([str(self.tmp)])
+        self.assertEqual(len(scan), 2)
+        self.assertIn("sha256", next(iter(scan.values())))
+
+
+class TestDiff(unittest.TestCase):
+    def setUp(self):
+        self.d = tempfile.TemporaryDirectory()
+        self.tmp = Path(self.d.name)
+        (self.tmp / "keep").write_text("keep")
+        (self.tmp / "gone").write_text("gone")
+        self.base = fim.scan_paths([str(self.tmp)])
+
+    def tearDown(self):
+        self.d.cleanup()
+
+    def test_modified_detected_with_preserved_mtime(self):
+        p = self.tmp / "keep"
+        st = p.stat()
+        p.write_text("TAMPERED")
+        os.utime(p, (st.st_atime, st.st_mtime))  # preserve mtime
+        new = fim.scan_paths([str(self.tmp)])
+        d = fim.diff(self.base, new)
+        self.assertIn((str(p), ["sha256"]), [(x, c) for x, c in d["modified"]])
+
+    def test_added_and_removed(self):
+        (self.tmp / "new").write_text("n")
+        (self.tmp / "gone").unlink()
+        d = fim.diff(self.base, fim.scan_paths([str(self.tmp)]))
+        self.assertIn(str(self.tmp / "new"), d["added"])
+        self.assertIn(str(self.tmp / "gone"), d["removed"])
+
+    def test_alerts_have_sids(self):
+        (self.tmp / "x").write_text("x")
+        d = fim.diff(self.base, fim.scan_paths([str(self.tmp)]))
+        alerts = list(fim.diff_alerts(d))
+        sids = {a.sid for a in alerts}
+        self.assertIn(fim.SID_ADDED, sids)
+
+
+class TestPacmanParse(unittest.TestCase):
+    SAMPLE = """\
+warning: qbittorrent: /usr/bin/qbittorrent (SHA256 checksum mismatch)
+openssh: /usr/bin/ssh (Permissions mismatch)
+coreutils: 105 total files, 1 altered files
+warning: No package owns /usr/local/bin/custom
+bar: /etc/bar.conf (Modification time mismatch)
+"""
+
+    def test_parse(self):
+        hits = fim.parse_pacman_verify(self.SAMPLE)
+        paths = {p for _pkg, p, _r in hits}
+        self.assertIn("/usr/bin/qbittorrent", paths)
+        self.assertIn("/usr/bin/ssh", paths)
+        # mtime-only and summary/no-owner lines are ignored
+        self.assertNotIn("/etc/bar.conf", paths)
+        self.assertEqual(len(hits), 2)
+
+    def test_pkg_alert_is_critical(self):
+        # exercise the alert builder via a monkeypatched verify
+        orig = fim.pacman_verify
+        fim.pacman_verify = lambda timeout=600: [("openssh", "/usr/bin/ssh", "SHA256 checksum mismatch")]
+        try:
+            alerts = list(fim.pacman_verify_alerts())
+        finally:
+            fim.pacman_verify = orig
+        self.assertEqual(alerts[0].severity, Severity.CRITICAL)
+        self.assertEqual(alerts[0].sid, fim.SID_PKG)
+
+
+if __name__ == "__main__":
+    unittest.main()