Re-architect Sentinel as a zero-dependency Python package

Port the bash prototype to a structured, testable Python codebase while
preserving the same control loop and the seven detection signatures. The bash
implementation stays in src/ as the regression oracle.

Highlights:
- enodia_sentinel/ package (stdlib only — no runtime deps):
  - detectors/ : one pure function per signature, detect(state, cfg)->Alerts
  - system.py  : SystemState — one cached /proc + ss snapshot per sweep,
                 fully injectable so detectors are unit-testable
  - daemon.py  : sweep loop, in-process cooldown dedup, threaded snapshot
                 capture, and a SUID filesystem scan moved OFF the loop
                 thread onto a slow background cadence
  - snapshot.py: forensic text + JSON sidecar with per-signature IR guidance
  - config.py  : dataclass config via TOML + env overrides
  - netutil.py : public-IP / CIDR logic via stdlib ipaddress
- tests/ : 25 stdlib-unittest cases (no root, no /proc, no ss needed)
- TOML config, launcher wrapper, Makefile (pip-free install), hardened
  systemd unit (env-resolved ExecStart), updated PKGBUILD, rewritten README

Performance: per-sweep cost ~200 ms (shared cached state); the multi-second
SUID walk no longer blocks detection. scandir-based walk replaces os.walk.

Verified on Arch: all 7 detectors fire on red-team drills (reverse_shell,
ld_preload, deleted_exe, new_listener, new_suid confirmed live end-to-end),
no false positives on a clean sweep, 25/25 tests pass.

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

tests/test_netutil.py

@@ -0,0 +1,51 @@
+import unittest
+
+from enodia_sentinel import netutil
+
+
+class TestPublicIP(unittest.TestCase):
+    def test_public(self):
+        self.assertTrue(netutil.is_public_ip("8.8.8.8"))
+        self.assertTrue(netutil.is_public_ip("1.1.1.1"))
+
+    def test_private_and_special(self):
+        for addr in ("10.0.0.5", "192.168.1.1", "172.16.0.1",
+                     "127.0.0.1", "169.254.1.1", "100.64.0.1", "::1"):
+            self.assertFalse(netutil.is_public_ip(addr), addr)
+
+    def test_zone_and_brackets(self):
+        self.assertFalse(netutil.is_public_ip("10.2.0.2%proton0"))
+        self.assertFalse(netutil.is_public_ip("[::1]"))
+
+    def test_garbage(self):
+        self.assertFalse(netutil.is_public_ip("not-an-ip"))
+        self.assertFalse(netutil.is_public_ip(""))
+
+
+class TestCidr(unittest.TestCase):
+    def test_in_and_out(self):
+        cidrs = ["203.0.113.0/24", "10.0.0.0/8"]
+        self.assertTrue(netutil.ip_in_cidrs("203.0.113.55", cidrs))
+        self.assertTrue(netutil.ip_in_cidrs("10.9.9.9", cidrs))
+        self.assertFalse(netutil.ip_in_cidrs("8.8.8.8", cidrs))
+
+    def test_empty_list(self):
+        self.assertFalse(netutil.ip_in_cidrs("8.8.8.8", []))
+
+
+class TestSplitHostPort(unittest.TestCase):
+    def test_ipv4(self):
+        self.assertEqual(netutil.split_host_port("1.2.3.4:443"), ("1.2.3.4", "443"))
+
+    def test_ipv6_bracket(self):
+        self.assertEqual(netutil.split_host_port("[2001:db8::1]:22"),
+                         ("2001:db8::1", "22"))
+
+    def test_zone(self):
+        host, port = netutil.split_host_port("10.2.0.2%proton0:61877")
+        self.assertEqual(host, "10.2.0.2%proton0")
+        self.assertEqual(port, "61877")
+
+
+if __name__ == "__main__":
+    unittest.main()