Use @property/@setter for writable mathutils properties

Writable mathutils properties (Vector, Euler, etc.) were typed as plain
attributes with a union type, causing the getter to also return the union.
This made `obj.location.x` fail type checking since Sequence[float] has
no `.x`. Now these properties use @property for the getter (returning the
concrete mathutils type) and @setter accepting the wider union.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/test_generate_stubs.py

@@ -420,6 +420,41 @@ class TestGeneratePropertyStub(unittest.TestCase):
         self.assertIn("def type(self) -> str:", result)
         self.assertIn("...", result)
 
+    def test_setter_type_generates_property_with_setter(self) -> None:
+        prop: PropertyData = {
+            "name": "location",
+            "type": "mathutils.Vector",
+            "is_readonly": False,
+            "description": "Object location.",
+            "setter_type": "mathutils.Vector | Sequence[float]",
+        }
+        result = generate_property_stub(prop)
+        self.assertIn("@property", result)
+        self.assertIn("def location(self) -> mathutils.Vector:", result)
+        self.assertIn('"""Object location."""', result)
+        self.assertIn("@location.setter", result)
+        self.assertIn(
+            "def location(self, value: mathutils.Vector | Sequence[float]) -> None: ...",
+            result,
+        )
+
+    def test_setter_type_no_description(self) -> None:
+        prop: PropertyData = {
+            "name": "rotation",
+            "type": "mathutils.Euler",
+            "is_readonly": False,
+            "description": "",
+            "setter_type": "mathutils.Euler | Sequence[float]",
+        }
+        result = generate_property_stub(prop)
+        self.assertIn("@property", result)
+        self.assertIn("def rotation(self) -> mathutils.Euler:", result)
+        self.assertIn("@rotation.setter", result)
+        self.assertIn(
+            "def rotation(self, value: mathutils.Euler | Sequence[float]) -> None: ...",
+            result,
+        )
+
 
 class TestGenerateMethodStub(unittest.TestCase):
     def test_simple_method(self) -> None: