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

@@ -177,23 +177,33 @@ DistanceType getDistanceType(App::DocumentObject* joint)
         }
 
         // One side is a datum plane, the other has a real element type.
-        // Ensure the datum (plane) side is Reference1 for consistent
-        // convention (face/plane first), matching the existing swap logic.
+        // 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 (!datum1) {
-            swapJCS(joint);
-        }
 
-        if (otherType == "Vertex") {
-            return DistanceType::PointPlane;
-        }
-        if (otherType == "Edge") {
+        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;
         }
-        if (otherType == "Face") {
-            return DistanceType::PlanePlane;
-        }
-        // Unknown element + datum plane → best-effort planar
+
+        // Face + datum or unknown + datum → PlanePlane.
+        // No swap needed: PlanarConstraint is symmetric (uses both
+        // z_i and z_j), and preserving the original Reference order
+        // keeps the initial Placement values consistent so the solver
+        // stays in the correct orientation branch.
         return DistanceType::PlanePlane;
     }