fix: preserve public k-point reader contracts and kvec_c_full after aa6fc43

- read_listed_kpoints (Cartesian/Direct) and interpolate_k_between are
  public, independently-usable readers (KlistTest.ReadKpointsCartesian,
  ReadKpointsLineCartesian) that must produce spin-doubled containers
  immediately, without requiring a later set_kup_and_kdw() call; revert
  their allocation back to nkstot*spin_mult. Only Monkhorst_Pack (which
  triggered the original build_kstars crash and has no such contract)
  keeps the deferred, undoubled allocation.
- set_kup_and_kdw() now resizes only kvec_c/kvec_d/wk/ngk/isk for the
  down-spin copy it is about to append, instead of calling renew() (which
  also resizes kvec_c_full). kvec_c_full must keep holding the original,
  un-reduced full-BZ mesh for later consumers (e.g. Ewald_Vq); renew()
  was truncating it to the symmetry-reduced, spin-doubled size.

Verified: MODULE_CELL_klist_test (34/34), MODULE_CELL_reciprocal_grid_test
(10/10), MODULE_CELL_qlist_test (16/16), MODULE_CELL_klist_test_para1
(2/2) all pass; 17_DS_DFTU/03_LCAO_DFTU_S2_Z with symmetry=1 still
produces the same energy as before this fix.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019omhQohPCePyJnLiWnG6Fa

Author: maki49

source/source_cell/klist.cpp

@@ -335,14 +335,14 @@ bool K_Vectors::read_listed_kpoints(std::ifstream& ifk, const std::string& kword
 {
     if (kword == "Cartesian" || kword == "C") // Cartesian coordinates
     {
-        this->renew(nkstot); // mohan fix bug 2009-09-01; spin_mult doubling done later by set_kup_and_kdw()
+        this->renew(nkstot * this->spin_mult); // mohan fix bug 2009-09-01
         KListIO::read_kpt_list(ifk, nkstot, this->kvec_c, this->wk);
         this->kc_done = true;
         return true;
     }
     if (kword == "Direct" || kword == "D") // Direct coordinates
     {
-        this->renew(nkstot); // mohan fix bug 2009-09-01; spin_mult doubling done later by set_kup_and_kdw()
+        this->renew(nkstot * this->spin_mult); // mohan fix bug 2009-09-01
         KListIO::read_kpt_list(ifk, nkstot, this->kvec_d, this->wk);
         this->kd_done = true;
         return true;
@@ -395,7 +395,7 @@ void K_Vectors::interpolate_k_between(std::ifstream& ifk, std::vector<ModuleBase
     const KListIO::LineK line = KListIO::interp_line(ifk, this->nkstot);
 
     this->nkstot = line.nks_total;
-    this->renew(this->nkstot); // mohan fix bug 2009-09-01; spin_mult doubling done later by set_kup_and_kdw()
+    this->renew(this->nkstot * this->spin_mult); // mohan fix bug 2009-09-01
 
     for (int i = 0; i < this->nkstot; i++)
     {
@@ -446,11 +446,17 @@ void K_Vectors::set_kup_and_kdw(std::ofstream& ofs_running)
 {
     ModuleBase::TITLE("K_Vectors", "setup_kup_and_kdw");
 
-    // grow the containers to make room for the down-spin copy expand_spin_kpoints()
-    // is about to append (indices [nkstot, 2*nkstot)); done here, on demand, rather
-    // than speculatively reserved earlier, so nothing upstream of this point ever
-    // sees kvec_d/kvec_c/wk/isk padded with not-yet-meaningful placeholder entries.
-    this->renew(this->nkstot * this->spin_mult);
+    // grow the containers expand_spin_kpoints() is about to append the down-spin
+    // copy into (indices [nkstot, 2*nkstot)). Resize only these; NOT via renew(),
+    // which would also resize kvec_c_full -- that one must keep holding the
+    // original, un-doubled, un-symmetry-reduced full-BZ mesh for later consumers
+    // (e.g. Ewald_Vq) regardless of what nkstot has become by this point.
+    const int nkstot_spin = this->nkstot * this->spin_mult;
+    this->kvec_c.resize(nkstot_spin);
+    this->kvec_d.resize(nkstot_spin);
+    this->wk.resize(nkstot_spin);
+    this->ngk.resize(nkstot_spin);
+    this->isk.resize(nkstot_spin);
 
     KListIO::expand_spin_kpoints(this->spin_mult,
                                  this->kvec_c,