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/App/AssemblyUtils.cpp

@@ -164,6 +164,49 @@ DistanceType getDistanceType(App::DocumentObject* joint)
     auto* obj1 = getLinkedObjFromRef(joint, "Reference1");
     auto* obj2 = getLinkedObjFromRef(joint, "Reference2");
 
+    // Datum planes (App::Plane) have empty element types because their
+    // sub-name ends with "." and yields no Face/Edge/Vertex element.
+    // Detect them here and classify before the main geometry chain,
+    // which cannot handle the empty element type.
+    const bool datum1 = type1.empty() && obj1 && obj1->isDerivedFrom<App::Plane>();
+    const bool datum2 = type2.empty() && obj2 && obj2->isDerivedFrom<App::Plane>();
+
+    if (datum1 || datum2) {
+        if (datum1 && datum2) {
+            return DistanceType::PlanePlane;
+        }
+
+        // One side is a datum plane, the other has a real element type.
+        // For PointPlane/LinePlane, the solver's PointInPlaneConstraint
+        // reads the plane normal from marker_j (Reference2).  Unlike
+        // real Face+Vertex joints (where both Placements carry the
+        // face normal from findPlacement), datum planes only carry
+        // their normal through computeMarkerTransform.  So the datum
+        // must end up on Reference2 for the normal to reach marker_j.
+        //
+        // For PlanePlane the convention matches the existing Face+Face
+        // path (plane on Reference1).
+        const auto& otherType = datum1 ? type2 : type1;
+
+        if (otherType == "Vertex" || otherType == "Edge") {
+            // Datum must be on Reference2 (j side).
+            if (datum1) {
+                swapJCS(joint);  // move datum from Ref1 → Ref2
+            }
+            if (otherType == "Vertex") {
+                return DistanceType::PointPlane;
+            }
+            return DistanceType::LinePlane;
+        }
+
+        // Face + datum or unknown + datum → PlanePlane
+        // Datum on Reference1 for consistency with Face+Face path.
+        if (!datum1) {
+            swapJCS(joint);  // move datum from Ref2 → Ref1
+        }
+        return DistanceType::PlanePlane;
+    }
+
     if (type1 == "Vertex" && type2 == "Vertex") {
         return DistanceType::PointPoint;
     }