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/AssemblyObject.cpp

@@ -1115,10 +1115,19 @@ KCSolve::SolveContext AssemblyObject::buildSolveContext(
                         break;
 
                     default:
+                        FC_WARN("Assembly : Distance joint '" << joint->getFullName()
+                                << "' — unhandled DistanceType "
+                                << distanceTypeName(distType)
+                                << ", falling back to Planar");
                         kind = KCSolve::BaseJointKind::Planar;
                         params.push_back(distance);
                         break;
                 }
+
+                FC_LOG("Assembly : Distance joint '" << joint->getFullName()
+                       << "' — DistanceType=" << distanceTypeName(distType)
+                       << ", kind=" << static_cast<int>(kind)
+                       << ", distance=" << distance);
                 break;
             }
             default: