fix(partition): 🔇 collapse identical COMMPLACE repeats

[diagonal-hamiltonian]

🤖 AI text below 🤖

Summary

Follow-up to #269, which made the COMMPLACE line unconditional at review request. That change is
correct and I am not walking it back — but making it unconditional exposed its volume, and the
volume is a defect:

One serial 585-test Python run emits 437 COMMPLACE lines, and all 437 are the same string.

A process has one placement. Reporting it 437 times is noise, not data, and it is on main now.

fix/commplace-collapse-repeats f87b662, 2 commits on 34db77d, 4 files +42/−8.

Why #269 did not catch this

Because it measured with pytest capturing. #269's body states that "the whole Python suite emits
nothing" — that reading came from a run without -s, and pytest's fd capture swallows C++ stderr.
Same arm, same command, -s the only difference:

	COMMPLACE lines
pytest … -q	0
pytest … -q -s	1

The zero was capture, not silence. Any future measurement of this line needs -s, and the probe
script now says so in a comment rather than leaving the next person to rediscover it.

The fix

place_line_is_new(line) returns true the first time a line is seen and whenever it differs from the
previous call; PartitionGroup writes only when it does.

Three choices in it worth stating, because the obvious implementations are each wrong in a way:

• It compares the whole line, not a once-flag. A std::call_once would report the first
  placement only. A process that builds an alone propagator before an MPI one has two placements
  and both are worth saying; a changed line still gets through.
• It is a non-template free function in CpuTopology.cpp, not a static local in
  PartitionGroup<NumModes>. A static inside the template gets one guard per NumModes
  instantiation, so the collapse would leak one line per distinct mode count.
• It returns a bool rather than writing. The write stays at the call site, and the collapse is
  testable without capturing stderr — which is exactly the instrument that hid the problem.

The second commit takes the argument by std::string_view (SonarQube cpp:S6009 on the first): the
function only compares and assigns, so it never needed to bind a std::string.

No knob, no behaviour change to what a distinct placement reports, and nothing branches on it.

Measurement

Arm md5 f34e996d. The counts are the point of the PR, so they are the measurement:

	before (34db77d)	after (d7ff600)
Python suite, 585 tests, -s	437 lines, 1 distinct	1
C++ unit binary	19 lines, 1 distinct	1

Gates on f34e996d: ctest -L unit 239/239, -L serial 238/238, job COMPLETED 0:0. The unit
count is one above main's 238 because of the new case. Python suite 585 passed, 8 deselected.
The string_view commit is a signature change only; its case still asserts 6 of 6 and the counts
above re-measured identically.

CI: all 16 checks green, clang-tidy 0 unsuppressed warnings, SonarQube 0 open issues.

The new case asserts 6 of 6 (--report_level=detailed) and covers the two failure modes separately:
a repeat is suppressed, and a changed line is not — the second is what a call_once implementation
would fail.

Ground-truth multi-rank behaviour is unchanged and re-verified with no env var set; the collapse is
per process, so co-located ranks still each report:

--cpu-bind=cores  COMMPLACE rank=0 node_rank=0 node_size=2 masks=private cpus=64  node_cpus=128 cpu_list=0-127
                  COMMPLACE rank=1 node_rank=1 node_size=2 masks=private cpus=64  node_cpus=128 cpu_list=0-127
--cpu-bind=none   COMMPLACE rank=0 node_rank=0 node_size=2 masks=shared  cpus=128 node_cpus=128 cpu_list=0-127
                  COMMPLACE rank=1 node_rank=1 node_size=2 masks=shared  cpus=128 node_cpus=128 cpu_list=0-127

These runs are not reproducible from the diff: they need the Deucalion harness, which is not in this
repo.

Checklist

• Tests added or updated to cover the changes
• Documentation updated (docstrings, docs/, CONTRIBUTING.md) if needed
• CHANGELOG / release notes updated if applicable

Docs unchanged on purpose: parallelism.mdx already says the line is written per rank at propagator
construction with no knob, and that stays true.

AI/LLM disclosure

• I did not use LLM tooling, or used it only privately for ideation
• I used the following tool to help write this PR description: Claude Code
• I used the following tool to generate or modify code: Claude Code

Important

By opening this PR I confirm that I have read CONTRIBUTING.md and I agree to the terms of the Contributor License Agreement.

Warning

If you're contributing on behalf of your employer, contact cla@algorithmiq.fi to arrange a Corporate CLA.

[github-actions]

Docs preview: https://pr-286.monoprop-docs.pages.dev

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.70%. Comparing base (9888faa) to head (363ae4a).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #286   +/-   ##
=======================================
  Coverage   97.70%   97.70%           
=======================================
  Files          14       14           
  Lines         742      742           
  Branches       98       98           
=======================================
  Hits          725      725           
  Misses         12       12           
  Partials        5        5           

Flag	Coverage Δ
cpp	97.70% <ø> (ø)

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
90.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud