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/AssemblyUtils.cpp

@@ -591,6 +591,19 @@ App::DocumentObject* getObjFromRef(App::DocumentObject* comp, const std::string&
         if (obj->isDerivedFrom<App::Part>() || obj->isLinkGroup()) {
             continue;
         }
+        else if (obj->isDerivedFrom<App::LocalCoordinateSystem>()) {
+            // Resolve LCS → child datum element (e.g. Origin → XY_Plane)
+            auto nextIt = std::next(it);
+            if (nextIt != names.end()) {
+                for (auto* child : obj->getOutList()) {
+                    if (child->getNameInDocument() == *nextIt
+                        && child->isDerivedFrom<App::DatumElement>()) {
+                        return child;
+                    }
+                }
+            }
+            return obj;
+        }
         else if (obj->isDerivedFrom<PartDesign::Body>()) {
             return handlePartDesignBody(obj, it);
         }