ADR-013 Phase 2: import guard round-4 fix (init-less dir .py sibling)

tests/test_plugin_importguard.py

@@ -167,6 +167,36 @@ def test_planted_sourceless_pyc_refused(guarded):
         guard.find_spec(f'plugins.{name}.models.evil')
 
 
+def test_tampered_py_with_sibling_dir_refused(guarded):
+    """Round 4: a signed foo.py tampered + an empty foo/ dir next to it must be
+    refused. FileFinder would load the file over the init-less dir, so the guard
+    must hash-gate the sibling .py instead of returning None for the dir."""
+    guard, plugins_dir, priv, name = guarded
+    pdir = _write_pkg_plugin(plugins_dir, name)
+    (pdir / 'models' / 'device.py').write_text('MARKER = "signed"\n')
+    _stamp(pdir, priv, name)
+
+    # attacker tampers the signed device.py and adds an empty sibling device/
+    (pdir / 'models' / 'device.py').write_text('MARKER = "evil"\n')
+    (pdir / 'models' / 'device').mkdir()
+
+    with pytest.raises(PluginVerificationError):
+        guard.find_spec(f'plugins.{name}.models.device')
+
+
+def test_unsigned_py_with_sibling_dir_refused(guarded):
+    """Symmetric variant: a never-signed evil.py next to evil/ is refused too."""
+    guard, plugins_dir, priv, name = guarded
+    pdir = _write_pkg_plugin(plugins_dir, name)
+    _stamp(pdir, priv, name)
+
+    (pdir / 'models' / 'evil.py').write_text('MARKER = "evil"\n')
+    (pdir / 'models' / 'evil').mkdir()
+
+    with pytest.raises(PluginVerificationError):
+        guard.find_spec(f'plugins.{name}.models.evil')
+
+
 def test_absent_module_defers(guarded):
     """A genuinely-absent module returns None (normal ModuleNotFoundError)."""
     guard, plugins_dir, priv, name = guarded