refactor(source_cell): fold KVectorUtils shim back into K_Vectors members

The KVectorUtils free functions were a migration-period compatibility
shim (2026-08-14); with all call sites now migrated, the shim only
added a second calling convention and two friend declarations.

- Move the real logic into K_Vectors members, line-by-line unchanged:
  set_after_vc (public, was KVectorUtils::set_after_vc) and mpi_k
  (private, __MPI-guarded, was KVectorUtils::kvec_mpi_k)
- Delete the four 1-line forwarders, of which kvec_d2c/kvec_c2d had no
  callers left, and delete k_vector_utils.h/.cpp entirely
- Mechanical call-site updates: klist.cpp internal calls go straight
  to the members, esolver_fp.cpp uses kv.set_after_vc(), tests use
  kv-> methods (kvec_ibz_kpoint -> reduce_by_symmetry with the member
  parameter order), 12 test CMakeLists.txt and Makefile.Objects drop
  the k_vector_utils.cpp registration
- Drop the two K_Vectors friend declarations; update two comments that
  referenced KVectorUtils::ibz_kpoint

Global dependency budget: k_vector_utils.cpp contributed 12 GlobalV
touches; the logic now lives inside the class that already carries the
k-list globals, so the PR-level count decreases.

Verified: make -j 30 (100%, incl. external module test targets); ctest
MODULE_CELL_{klist,reciprocal_grid,qlist}_test and klist_test_para1/
para4 5/5 passed (MPI path covers the moved mpi_k); grep confirms no
KVectorUtils references remain outside the two historical notes in
reciprocal_grid.h/.cpp.

Author: abacus_fixer

source/Makefile.Objects

@@ -211,7 +211,6 @@ OBJS_CELL=atom_pseudo.o\
     print_cell.o\
     setup_nonlocal.o\
     klist.o\
-    k_vector_utils.o\
     reciprocal_grid.o\
     cell_index.o\
     cell_tools.o\

source/source_cell/CMakeLists.txt

@@ -33,7 +33,6 @@ add_library(
     read_stru.cpp
     print_cell.cpp
     read_atom_species.cpp
-    k_vector_utils.cpp
     sep.cpp
     sep_cell.cpp
     qlist.cpp