feat(solver): refactor AssemblyObject to use IKCSolver interface (#295)

Rewire AssemblyObject to call through KCSolve::IKCSolver instead of
directly manipulating OndselSolver ASMT types.

Key changes:
- Remove all 30+ #include <OndselSolver/*> from AssemblyObject.cpp
- Remove MbDPartData, objectPartMap, mbdAssembly members
- Add solver_ (IKCSolver), lastResult_ (SolveResult), partIdToObjs_ maps
- New buildSolveContext() builds SolveContext from FreeCAD document objects
  with JointType/DistanceType -> BaseJointKind decomposition
- New computeMarkerTransform() replaces handleOneSideOfJoint()
- New computeRackPinionMarkers() replaces getRackPinionMarkers()
- Rewrite solve/preDrag/doDragStep/postDrag/generateSimulation to call
  through IKCSolver interface
- Rewrite setNewPlacements/validateNewPlacements to use SolveResult
- Rewrite updateSolveStatus to use ConstraintDiagnostic
- Add export_native() to IKCSolver for ASMT export support
- Register OndselAdapter at Assembly module init in AppAssembly.cpp
- Remove OndselSolver from Assembly_LIBS (linked transitively via KCSolve)

Assembly module now has zero OndselSolver includes. All solver coupling
is confined to KCSolve/OndselAdapter.cpp.

src/Mod/Assembly/Solver/OndselAdapter.cpp

@@ -784,4 +784,13 @@ std::vector<ConstraintDiagnostic> OndselAdapter::diagnose(const SolveContext& ct
     return extract_diagnostics();
 }
 
+// ── Native export ──────────────────────────────────────────────────
+
+void OndselAdapter::export_native(const std::string& path)
+{
+    if (assembly_) {
+        assembly_->outputFile(path);
+    }
+}
+
 }  // namespace KCSolve