DeltaP cleanup: fix run_deltap_lambda_loop + deprecation markers

#3 run_deltap_lambda_loop: add deltap_corr gate, document Phase D plan
#4 GPU stubs: already have explanatory comments (d89800f)
#5 deltap_solver.h: mark deprecated, unused by any code path
#6 deltap_common.h: note 3/4 functions unused, kept for future refactoring

BN LCAO + PW regression pass.

Author: dyzheng

source/source_esolver/deltap_common.h

@@ -7,6 +7,9 @@
  * Separates the linear algebra of lambda updates, constraint matrix handling,
  * and residual computation from the basis-specific Hamiltonian application
  * and gamma computation.
+ *
+ * Currently only compute_dp_escon() is used (by LCAO).  The other functions
+ * are kept for future constraint-matrix / total-mode refactoring.
  */
 
 #include <vector>
@@ -27,6 +30,9 @@ namespace deltap_common {
  * @param mixing       Damping factor (0 = no mixing, 1 = full step).
  * @param lambda_out   Effective per-atom lambda [n_atoms] (output).
  * @return             Max residual norm (for convergence check).
+ * @note This function is currently unused — both LCAO and PW paths
+ *   implement lambda updates inline.  Kept for future total/constraint-matrix
+ *   refactoring.
  */
 inline double update_lambda(
     const std::vector<std::vector<double>>& C,

source/source_esolver/deltap_solver.h

@@ -3,6 +3,11 @@
 /**
  * @file deltap_solver.h
  * @brief Solver callback interface for DeltaP inner loop.
+ * @deprecated This file is not used by any code path.  The inner-loop
+ *   logic for both LCAO and PW is implemented directly in the ESolver
+ *   classes (esolver_ks_lcao.cpp, deltap_pw.cpp).  This file may be
+ *   removed in a future cleanup.
+ */
  *
  * The inner BFGS loop needs to apply trial lambda values and re-solve
  * the KS equation with frozen charge density.  This is basis-specific:

source/source_pw/module_pwdft/deltap_pw.cpp

@@ -75,10 +75,15 @@ bool run_deltap_lambda_loop(const int iter,
                             const double drho,
                             const Input_para& inp)
 {
-    if (!inp.deltap_switch)
+    if (!inp.deltap_switch || !inp.deltap_corr)
         return false;
 
     set_deltap_pw_active(true);
+
+    // Phase A: no inner loop — always run normal HSolver.
+    // Lambda is updated in deltap_iter_finish() after charge convergence.
+    // Phase D (deferred): inner BFGS with subspace diag, would return true
+    //   to skip HSolver and run cal_hs_subspace + diag_responce instead.
     return false;
 }