feat(assembly): fixed reference planes + solver docs

Assembly Origin Planes:
- AssemblyObject::setupObject() relabels origin planes to
  Top (XY), Front (XZ), Right (YZ) on assembly creation
- CommandCreateAssembly.py makes origin planes visible by default
- AssemblyUtils.cpp getObjFromRef() resolves LocalCoordinateSystem
  to child datum elements for joint references to origin planes
- TestAssemblyOriginPlanes.py: 9 integration tests covering
  structure, labels, grounding, reference resolution, solver,
  and save/load round-trip

Solver Documentation:
- docs/src/solver/: 7 new pages covering architecture overview,
  expression DAG, constraints, solving algorithms, diagnostics,
  assembly integration, and writing custom solvers
- docs/src/SUMMARY.md: added Kindred Solver section

src/Mod/Assembly/App/AssemblyObject.cpp

@@ -30,6 +30,7 @@
 
 #include <App/Application.h>
 #include <App/Datums.h>
+#include <App/Origin.h>
 #include <App/Document.h>
 #include <App/DocumentObjectGroup.h>
 #include <App/FeaturePythonPyImp.h>
@@ -106,6 +107,24 @@ AssemblyObject::AssemblyObject()
 
 AssemblyObject::~AssemblyObject() = default;
 
+void AssemblyObject::setupObject()
+{
+    App::Part::setupObject();
+
+    // Relabel origin planes with assembly-friendly names (SolidWorks convention)
+    if (auto* origin = getOrigin()) {
+        if (auto* xy = origin->getXY()) {
+            xy->Label.setValue("Top");
+        }
+        if (auto* xz = origin->getXZ()) {
+            xz->Label.setValue("Front");
+        }
+        if (auto* yz = origin->getYZ()) {
+            yz->Label.setValue("Right");
+        }
+    }
+}
+
 PyObject* AssemblyObject::getPyObject()
 {
     if (PythonObject.is(Py::_None())) {
@@ -144,14 +163,39 @@ void AssemblyObject::onChanged(const App::Property* prop)
 
 // ── Solver integration ─────────────────────────────────────────────
 
+void AssemblyObject::resetSolver()
+{
+    solver_.reset();
+}
+
 KCSolve::IKCSolver* AssemblyObject::getOrCreateSolver()
 {
     if (!solver_) {
-        solver_ = KCSolve::SolverRegistry::instance().get("ondsel");
+        ParameterGrp::handle hGrp = App::GetApplication().GetParameterGroupByPath(
+            "User parameter:BaseApp/Preferences/Mod/Assembly");
+        std::string solverName = hGrp->GetASCII("Solver", "");
+        solver_ = KCSolve::SolverRegistry::instance().get(solverName);
+        // get("") returns the registry default (first registered solver)
     }
     return solver_.get();
 }
 
+KCSolve::SolveContext AssemblyObject::getSolveContext()
+{
+    partIdToObjs_.clear();
+    objToPartId_.clear();
+
+    auto groundedObjs = getGroundedParts();
+    if (groundedObjs.empty()) {
+        return {};
+    }
+
+    std::vector<App::DocumentObject*> joints = getJoints(false);
+    removeUnconnectedJoints(joints, groundedObjs);
+
+    return buildSolveContext(joints);
+}
+
 int AssemblyObject::solve(bool enableRedo, bool updateJCS)
 {
     ensureIdentityPlacements();