MPI final: operator outside chain + rank-0 contributeHk

DeltaPOperator created but NOT added to ops chain (avoids chain
traversal crash on non-root MPI ranks with distributed k-points).
HK correction applied from iter_finish on rank 0 only.

NP=1: full constraint works
NP=2: measurement works, constraint pending HK path fix

Author: dyzheng

source/source_esolver/esolver_ks_lcao.cpp

@@ -1411,6 +1411,19 @@ void ESolver_KS_LCAO<TK, TR>::deltap_update_lambda(UnitCell& ucell, const int it
         std::unordered_map<int, std::vector<std::complex<double>>> hk_corr;
         dp->compute_hk_correction(ucell, psi, lambda, hk_corr);
         dp_op->set_hk_correction(hk_corr);
+
+        // Apply HK correction on rank 0 only (operator not in chain for MPI)
+#ifdef __MPI
+        if (this->pv.comm() != MPI_COMM_NULL) {
+            int rank = 0;
+            MPI_Comm_rank(this->pv.comm(), &rank);
+            if (rank == 0)
+#endif
+                for (int ik = 0; ik < this->kv.get_nks(); ++ik)
+                    dp_op->contributeHk(ik);
+#ifdef __MPI
+        }
+#endif
     }
 }