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/IKCSolver.h

@@ -157,6 +157,12 @@ public:
         return true;
     }
 
+    /// Export solver-native debug/diagnostic file (e.g. ASMT for OndselSolver).
+    /// Default: no-op.  Requires a prior solve() or run_kinematic() call.
+    virtual void export_native(const std::string& /*path*/)
+    {
+    }
+
     /// Whether this solver handles fixed-joint part bundling internally.
     /// When false, the caller bundles parts connected by Fixed joints
     /// before building the SolveContext. When true, the solver receives