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

@@ -199,11 +199,11 @@ DistanceType getDistanceType(App::DocumentObject* joint)
             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
-        }
+        // 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;
     }