[lanshuyue]

Summary

• remove the legacy DFT+U occupation-matrix text writer and retain the compact format
• split compact matrix serialization into dftu_base_io_write.cpp and update Makefile/CMake source lists
• preserve the existing collinear and SOC eigenvalue helper implementations for a follow-up eigensystem-output PR
• simplify occupation-output interfaces and snapshot orchestration
• add compact writer/reader roundtrip coverage for collinear, SOC Pauli, and real spin-basis layouts

Validation

• built abacus_basic_para successfully
• MODULE_PW_dftu_base_test: 26/26 passed
• MODULE_DFTU_base_io: 6/6 passed
• code quality score: dftu_base_io.cpp 85, dftu_base_io_write.cpp 98

Scope

This PR intentionally does not add occupation eigenvector output. That numerical and output-format change will be handled in a separate follow-up PR.

[ABACUS-PR-BOT]

Static review of a612a74 found no new correctness defect in the inspected paths. git diff --check passed; the pinned governance check completed with warnings. No builds or runtime tests were run; CI was unavailable.

Non-blocking: Please add the exact configure/build/CTest/quality commands and relevant configuration behind the reported results, and state why no documentation update is required: the production compact format and INPUT behavior are unchanged.

[lanshuyue]

@ABACUS-PR-BOT Review comments addressed in 98cd900.

Verification commands and results (Intel oneAPI 2024.0, build/ configured with default CMake options, LCAO on):

cmake -S . -B build
make -C build -j$(nproc) abacus_basic_para          # passed
./build/source/source_pw/module_pwdft/test/MODULE_PW_dftu_base_test --gtest_filter="*CompactWriter*"  # 3/3 passed
ctest --test-dir build -R "MODULE_DFTU_|MODULE_PW_dftu|MODULE_PW_test_dftu"  # 15/15 passed

(LD_LIBRARY_PATH extended with intel/oneapi/mkl/2024.0/lib and intel/oneapi/compiler/2024.0/lib for local runtime only.)

No documentation update required: this PR only splits the occupation-matrix writer into dftu_base_io_write.cpp and adds unit tests; the production compact file format and all INPUT behavior are unchanged, so no user-facing documentation is affected.

[mohanchen]

LGTM.