refactor(cell): split unitcell.cpp into index/stats/setup files

[DanielZhao0432]

Reminder

• I have read AGENTS.md and docs/developers_guide/agent_governance.md.
• I have linked an issue or explained why this PR does not need one.
• I have added adequate unit tests and/or case tests, or explained why not.
• I have listed the exact verification commands run and their results.
• I have described user-visible behavior changes, including INPUT parameter changes.
• I have explained core-module impact for ESolver, HSolver, ElecState, Hamilt, Operator, Psi, or other source/ changes.
• I have requested any needed governance exception below.

Linked Issue

No linked issue: this is a pure code-motion refactor plus new unit tests, with no behavior change.

Unit Tests and/or Case Tests for my changes

• Commands run:
  • cmake -B build -DBUILD_TESTING=ON -DENABLE_MPI=ON ... && cmake --build build -j30
  • OMP_NUM_THREADS=1 ctest --test-dir build -V -R MODULE_CELL_unitcell_index_test
  • OMP_NUM_THREADS=1 ctest --test-dir build -R MODULE_CELL
  • python3 tools/03_code_analysis/agent_governance_check.py --base upstream/develop --head HEAD --format text
  • python3 tools/03_code_analysis/code_quality_score.py <changed cpp/h files>
• Result summary: full build succeeded; new MODULE_CELL_unitcell_index_test 4/4 passed; all MODULE_CELL* tests 47/47 passed; governance check reported only non-blocking warnings (see Governance Notes); quality score 5/5 files passing (average 91.6, minimum 67 >= 60 pass line).
• New tests: source/source_cell/test/test_unitcell_index.cpp (named per the test_<module_name>.cpp rule), 4 TEST_F cases covering set_iat2itia (including the delete[]+new[] re-entry path) and set_iat2iwt with npol=1 and npol=2. The fixture uses a stack UnitCell with set_atom_flag=true; it does not manually free atoms (owned by ~UnitCell()) or iat2it/iat2ia (owned by the internal Statistics member), per AGENTS.md.

What's changed?

• Split source/source_cell/unitcell.cpp (349 lines) by functionality, code motion only, no logic changes:
  • unitcell.cpp keeps the constructor/destructor.
  • unitcell_index.cpp holds the indexing helpers set_iat2itia() and set_iat2iwt().
  • unitcell_stats.cpp holds the per-species counting helpers get_atom_Counts(), get_orbital_Counts(), get_lnchi_Counts().
  • unitcell_setup.cpp holds the setup routines setup_cell() and setup_from_input().
• CMake updated accordingly: the cell OBJECT library and the six test CMakeLists that compile unitcell.cpp directly (cell_simple_srcs, MODULE_CELL_read_atoms_helper_test, test_pw, source_md/test, module_deepks/test, module_pwdft/test x2) now register the three new files, so no target loses symbols.
• Added the unit test target MODULE_CELL_unitcell_index_test (links cell_info, plus ../magnetism.cpp for the Magnetism ctor/dtor symbols).

Governance Notes

• INPUT/docs changes: none; no INPUT parameter or user-visible behavior changed, so docs/parameters.yaml and docs/advanced/input_files/input-main.md do not require updates.
• Core module impact: confined to source_cell; all public interfaces (declarations in unitcell.h) are unchanged, so callers and existing mocks (e.g. test/support/mock_unitcell.cpp) are unaffected.
• The seven GlobalV warnings on unitcell_setup.cpp are migration-neutral (net_delta=0): the lines were moved verbatim from unitcell.cpp.
• Quality-score deductions on unitcell_setup.cpp (17-parameter setup_cell, GlobalV uses) are pre-existing debt carried by the moved code, not introduced by this PR.
• Exceptions requested: none.