feat: add distributed MdCell domain decomposition

[19hello]

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.

No linked issue. This PR is a scoped internal refactor in the staged parallel-MD series and does not change the INPUT interface.

Unit Tests and/or Case Tests for my changes

• Commands run:

  cmake -S /tmp/abacus-pr2-rework -B /tmp/abacus-pr2-rework-build \
    -DENABLE_MPI=ON -DBUILD_TESTING=ON -DENABLE_LCAO=OFF -DENABLE_LIBXC=OFF
  
  env CCACHE_DISABLE=1 cmake --build /tmp/abacus-pr2-rework-build \
    --target abacus_pw_para \
             MODULE_CELL_NEIGHBOR_neighbor_search \
             MODULE_CELL_NEIGHBOR_mdcell_migrate_mpi \
             MODULE_CELL_NEIGHBOR_distributed_mdcell_reader \
             MODULE_MD_LJ_pot -j4
  
  env LD_PRELOAD=/usr/lib/x86_64-linux-gnu/openblas-openmp/libopenblas.so.0 \
    OMP_NUM_THREADS=1 \
    ctest --test-dir /tmp/abacus-pr2-rework-build \
    -R 'MODULE_CELL_NEIGHBOR_(neighbor_search|mdcell_migrate_mpi|distributed_mdcell_reader_np4)$' \
    --output-on-failure
  
  env LD_PRELOAD=/usr/lib/x86_64-linux-gnu/openblas-openmp/libopenblas.so.0 \
    OMP_NUM_THREADS=1 \
    ctest --test-dir /tmp/abacus-pr2-rework-build \
    -R '^MODULE_MD_LJ_pot$' --output-on-failure
  
  cd tests/04_FF
  env LD_PRELOAD=/usr/lib/x86_64-linux-gnu/openblas-openmp/libopenblas.so.0 \
    OMP_NUM_THREADS=1 \
    bash ../integrate/Autotest.sh \
    -a /tmp/abacus-pr2-rework-build/abacus_pw_para \
    -f CASES_CPU.txt -r '^01_LJ_Anderson$' -n 2 -o 1
  
  python3 tools/03_code_analysis/agent_governance_check.py --staged
  git diff --check
  

• Result summary:

  • abacus_pw_para built successfully.
  • All 3 focused neighbor/MPI tests passed.
  • MODULE_MD_LJ_pot passed.
  • 2-rank 01_LJ_Anderson integration case passed all energy, force, and stress checks.
  • Staged governance check and whitespace check passed.

• Checks not run, with reason:

  • No full integration suite was run; this PR is limited to the distributed cell/neighbor infrastructure and its focused MPI and LJ regression coverage.

What's changed?

• Add MdCell as a distributed MD cell representation with owned atoms, ghost atoms, atom migration, and lattice updates.
• Add a distributed STRU reader for LJ-MD initialization.
• Extend neighbor-search infrastructure to work with UnitCell and MdCell.
• Remove obsolete AtomProvider and UnitCellLite abstractions.
• Update the existing LJ path to use UnitCell directly; this PR does not enable the new MdCell-based LJ solver path yet.
• Replace the obsolete neighbor MPI benchmark with focused migration and distributed-reader MPI tests.

Governance Notes

• INPUT/docs changes:

  • No INPUT parameter, parser, default-value, or user-visible documentation behavior changed. No updates to docs/parameters.yaml or input
    documentation are required.

• Core module impact:

  • Changes are limited to source_cell, neighbor-list infrastructure, and focused MD/LJ test linkage.
  • No ESolver behavior is changed beyond replacing its internal UnitCellLite adapter with direct UnitCell use.
  • MdCell-based solver execution, MD-driver dispatch, and end-to-end parallel-MD workflow integration are intentionally deferred to later PRs in
    this series.

• Exceptions requested:

  • None.
  • Header dependency warnings are intentional: MdCell owns LocalAtom vectors and exposes lattice-related value/reference types, so the complete
    declarations are required.