fix(assembly): classify datum plane references in Distance joints

When a Distance joint references a datum plane (XY_Plane, XZ_Plane,
YZ_Plane), getDistanceType() failed to recognize it because datum
plane sub-names yield an empty element type. This caused the fallback
to DistanceType::Other → BaseJointKind::Planar, which adds spurious
parallel-normal residuals that overconstrain the system.

For example, three vertex-to-datum-plane Distance joints produced
10 residuals (3×Planar) with 6 mutually contradictory orientation
constraints, causing the solver to find garbage least-squares
solutions.

Add early detection of App::Plane datum objects before the main
geometry classification chain. Datum planes are now correctly mapped:
- Vertex + DatumPlane → PointPlane → PointInPlane (1 residual)
- Edge + DatumPlane → LinePlane → LineInPlane
- Face/DatumPlane + DatumPlane → PlanePlane → Planar

src/Mod/Assembly/AssemblyTests/TestAssemblyOriginPlanes.py

@@ -191,6 +191,90 @@ class TestAssemblyOriginPlanes(unittest.TestCase):
         result = self.assembly.solve()
         self.assertEqual(result, 0, "Solve should succeed with origin plane joint")
 
+    # ── Distance joint to datum plane tests ────────────────────────
+
+    def test_distance_vertex_to_datum_plane_solves(self):
+        """Distance(0) joint: vertex → datum plane solves and pins position."""
+        origin = self._get_origin()
+        xy = origin.getXY()  # Top (Z normal)
+        xz = origin.getXZ()  # Front (Y normal)
+        yz = origin.getYZ()  # Right (X normal)
+
+        box = self._make_box(50, 50, 50)
+
+        # 3 Distance joints, each vertex→datum, distance=0.
+        # This should pin the box's Vertex1 (corner at local 0,0,0) to the
+        # origin, giving 3 PointInPlane constraints (1 residual each = 3 total).
+        for plane in [xy, xz, yz]:
+            joint = self._make_joint(
+                5,  # Distance
+                [box, ["Vertex1", "Vertex1"]],
+                [origin, [plane.Name + ".", plane.Name + "."]],
+            )
+            joint.Distance = 0.0
+
+        result = self.assembly.solve()
+        self.assertEqual(
+            result, 0, "Solve should succeed for vertex→datum Distance joints"
+        )
+
+        # The box's Vertex1 (at local 0,0,0) should be at the origin.
+        v = box.Placement.Base
+        self.assertAlmostEqual(v.x, 0.0, places=2, msg="X should be pinned to 0")
+        self.assertAlmostEqual(v.y, 0.0, places=2, msg="Y should be pinned to 0")
+        self.assertAlmostEqual(v.z, 0.0, places=2, msg="Z should be pinned to 0")
+
+    def test_distance_vertex_to_datum_plane_preserves_orientation(self):
+        """Distance(0) vertex→datum should not constrain orientation."""
+        origin = self._get_origin()
+        xy = origin.getXY()
+        xz = origin.getXZ()
+        yz = origin.getYZ()
+
+        # Start box with a known rotation (45° about Z).
+        rot = App.Rotation(App.Vector(0, 0, 1), 45)
+        box = self._make_box(50, 50, 50)
+        box.Placement = App.Placement(App.Vector(50, 50, 50), rot)
+
+        for plane in [xy, xz, yz]:
+            joint = self._make_joint(
+                5,
+                [box, ["Vertex1", "Vertex1"]],
+                [origin, [plane.Name + ".", plane.Name + "."]],
+            )
+            joint.Distance = 0.0
+
+        self.assembly.solve()
+
+        # 3 PointInPlane constraints pin position (3 DOF) but leave
+        # orientation free (3 DOF).  The solver should keep the original
+        # orientation since it's the lowest-energy solution from the
+        # initial placement.
+        dof = self.assembly.getLastDoF()
+        self.assertEqual(
+            dof, 3, "3 PointInPlane constraints should leave 3 DOF (orientation)"
+        )
+
+    def test_distance_face_to_datum_plane_solves(self):
+        """Distance(0) joint: face → datum plane solves (PlanePlane/Planar)."""
+        origin = self._get_origin()
+        xy = origin.getXY()
+
+        box = self._make_box(0, 0, 50)
+
+        # Face1 is the -Z face of a Part::Box.
+        joint = self._make_joint(
+            5,
+            [box, ["Face1", "Vertex1"]],
+            [origin, [xy.Name + ".", xy.Name + "."]],
+        )
+        joint.Distance = 0.0
+
+        result = self.assembly.solve()
+        self.assertEqual(
+            result, 0, "Solve should succeed for face→datum Distance joint"
+        )
+
     # ── Round-trip test ──────────────────────────────────────────────
 
     def test_save_load_preserves_labels(self):