[Zanthoxylum]

Summary

The oversized DFPT serial test files are split along the source
boundaries they exercise, with verbatim test moves and zero
production-code changes. After this PR no split test file exceeds
550 lines (was 1062 / 880 / 668), and every test file is
self-contained in gtest style: each .cpp carries its own fixture
class with only the members and helpers its cases need — no test
fixture headers.

Old file (deleted)	New files	Cases
dfpt_phon_serial_test.cpp (1062 lines)	test_dfpt_phon_ewald.cpp (ion_ion), test_dfpt_phon_elec.cpp (accumulate_electron + nlcc fill semantics), test_dfpt_phon_asr.cpp (the apply_asr family), test_dfpt_phon.cpp (diagonalize / LO-TO / sum rule / format)	20 = 3+4+7+6
dfpt_rho_serial_test.cpp (880 lines)	test_dfpt_rho.cpp (compute_drho / charge conservation / v_hartree_q / drhoc localization), test_dfpt_rho_mix.cpp (plain / Kerker / Broyden mixing family)	11 = 5+6
dfpt_q0_serial_test.cpp (668 lines)	test_dfpt_q0.cpp (build_vkb_dk / compute_eps / compute_born / star rotation), test_dfpt_q0_pos.cpp (pos_matrix kinetic + nonlocal velocity)	6 = 4+2
dfpt_xc_analytic_serial_test.cpp	helpers inlined back into the single file (550 lines: single-concern, single-source)	6 = 6

The pre-existing shared test utility dfpt_serial_fixture.{h,cpp}
(cell builders, real serial basis setup — compiled once per target
like dfpt_test_mocks.cpp) is untouched; each new file's fixture
derives from it and adds only what its family needs. The rho files
carry a file-local RNG in an anonymous namespace (single-translation-
unit determinism; no cross-TU sharing, hence no inline-variable
concerns).

Rationale

A test file that keeps growing is a signal that the source file under
test accumulates unrelated responsibilities ("god file"). The phon test
had grown to 1062 lines covering four disjoint families; this split
restores the one-file-one-concern mapping (coding rule 11 naming,
test_<module>.cpp), and the per-file fixtures keep every file
independent — editing one file's scaffolding never recompiles or
constrains its siblings.

What did NOT change

• No production source file is touched (only test_serial/ files and
  its CMakeLists.txt SOURCES lists; the CTest target names and link
  structure are unchanged).
• All test bodies are verbatim moves; beyond file headers the only
  adaptations are the per-file fixture splits, the file-local RNG
  (one reseeding call site updated to g_seed = 777u), and the
  dfpt_rho.h include living in test_dfpt_phon_elec.cpp alone.

Verification

Exact commands run (GCC 13 toolchain build dir build-gcc13, serial):

cmake -S . -B build-gcc13 && cmake --build build-gcc13 --target \
  MODULE_DFPT_phon_serial MODULE_DFPT_rho_serial MODULE_DFPT_q0_serial \
  MODULE_DFPT_xc_analytic_serial -j 20
ctest --test-dir build-gcc13 -R "MODULE_DFPT"          # 10/10 targets Passed

• Case-count guard per target via --gtest_list_tests:
  phon 20 = 20, rho 11 = 11, q0 6 = 6, xc_analytic 6 = 6 (identical
  case names, none added or lost).
• All four reworked targets rebuilt from deleted binaries (make exit
  code 0); full MODULE_DFPT suite re-run after every review round:
  10/10.
• python3 tools/03_code_analysis/agent_governance_check.py --base origin/develop --head HEAD: no findings beyond the standard
  documentation-sync notice.

Documentation

No docs change needed: test-only refactor with no INPUT, interface, or
behavior change.

Governance Exception

• Reason: test_dfpt_phon_asr.cpp and test_dfpt_rho_mix.cpp are
  named after the function family they exercise (the apply_asr
  family / the mix_drho stack) rather than after a current source
  file, because those families currently live inside dfpt_phon.cpp /
  dfpt_rho.cpp alongside unrelated concerns — rule 11's
  one-file-one-source mapping cannot be satisfied while the families
  share a source file.
• Scope: the two test file names only; every other new test file
  matches its source exactly (ewald / elec / q0 / q0_pos / rho).
• Risk: low — discoverability only; both files carry header
  comments naming the current source location of the family.
• Follow-up: the companion source splits refactor(dfpt): move the ASR family to dfpt_phon_asr.cpp #8260 (dfpt_phon_asr.cpp)
  and refactor(dfpt): move the rho mixing family to dfpt_rho_mix.cpp #8262 (dfpt_rho_mix.cpp) move those families into their own
  source files, after which the names match exactly (the rho test file
  already uses the post-split name); if either split is rejected, the
  test file is renamed to match its source instead.

[ABACUS-PR-BOT]

Static review found a C++17-only declaration in the shared RNG, plus two nonblocking governance/dependency suggestions. The case bodies, assertions and numerical helper formulas are preserved. No builds or runtime tests were run.