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.h

@@ -84,6 +84,7 @@ public:
         return "AssemblyGui::ViewProviderAssembly";
     }
 
+    void setupObject() override;
     App::DocumentObjectExecReturn* execute() override;
     void onChanged(const App::Property* prop) override;
     /* Solve the assembly. It will update first the joints, solve, update placements of the parts
@@ -98,10 +99,15 @@ public:
     void postDrag();
     void savePlacementsForUndo();
     void undoSolve();
+    void resetSolver();
     void clearUndo();
 
     void exportAsASMT(std::string fileName);
 
+    /// Build the assembly constraint graph without solving.
+    /// Returns an empty SolveContext if no parts are grounded.
+    KCSolve::SolveContext getSolveContext();
+
     bool validateNewPlacements();
     void setNewPlacements();
     static void redrawJointPlacements(std::vector<App::DocumentObject*> joints);