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/App/AppAssembly.cpp

@@ -26,6 +26,8 @@
 #include <Base/Interpreter.h>
 #include <Base/PyObjectBase.h>
 
+#include <Mod/Assembly/Solver/OndselAdapter.h>
+
 #include "AssemblyObject.h"
 #include "AssemblyLink.h"
 #include "BomObject.h"
@@ -54,6 +56,10 @@ PyMOD_INIT_FUNC(AssemblyApp)
     }
 
     PyObject* mod = Assembly::initModule();
+
+    // Register the built-in OndselSolver adapter with the solver registry.
+    KCSolve::OndselAdapter::register_solver();
+
     Base::Console().log("Loading Assembly module... done\n");