feat(partition): 📈 report the launcher's placement, and delete the pinning knob

[diagonal-hamiltonian]

🤖 AI text below 🤖

Summary

Adds a report-only diagnostic for the CPU placement the launcher actually gave each rank, and
deletes the knob that made pinning optional. feat/commplace-placement-report bf61814, 1 commit
on e0c528e, 9 files +464/−59.

1. monoprop_COMMPLACE reports the placement the launcher actually gave each rank. One
   COMMPLACE line per rank on stderr at propagator construction, naming the CPUs the launcher
   gave that rank and whether co-located ranks got disjoint masks (masks=private vs
   masks=shared); default off, and nothing below the flag is computed when it is off. The
   diagnostic earns a knob because --cpu-bind=none and --cpu-bind=cores give the same thread
   count and the same partition count, differing only in the mask the kernel enforces — a
   difference that has already cost a 1.45x tax nobody could see. A rank holding 16 of a host's
   128 CPUs is equally "Slurm gave me my own 16" and "eight of us share these 16", and sampling
   /proc cannot separate them: it describes the process you are in, never a peer rank on the
   same host. PartitionGroup already exchanges the masks to pick a placement, so this reports
   what that exchange saw. The flag gates printing only: the mask exchange is deliberately not
   gated on it, since the environment is per-rank, so a predicate over it is not rank-uniform, and
   one rank skipping a collective its peers entered is a hang, not a missing diagnostic.
   emit_place_line takes the flag rather than reading it, so a test binary launched without it
   set still reaches the emitting path.

2. monoprop_PARTITION_PINNING is deleted; pinning is now unconditional. The old parser
   matched only the first character against {0,f,F,n,N}. o is not in that set, so
   0/false/no worked while off, OFF and disabled all parsed as ON. The
   first-character match was deliberate and had a test asserting it — the bug is the vocabulary it
   produces. parse_env_flag now compares whole words, case-insensitively, against
   0|false|no|off. Deliberate consequence: bare f and n were falsey and are now truthy; a
   test pins that, and is mutation-verified — restoring first-character parsing fails it. No
   campaign ran on the wrong arm: no harness script ever set the variable, verified across the
   whole harness repo, so nothing needs re-auditing.

Not shipped: the placement deletion this was salvaged from. Its pre-registered falsifier was
abandon if >5% cost anywhere, and propagate[hubbard] at layout A/N=1 came in 2.90x slower, 10
of 10, with 11 of 12 layout-A tests slower. Refuted by its own criterion — which is why the knob
can be deleted rather than fixed. (Correcting an earlier draft of that work: main places at
both layouts, 128 at A and 16 at B.)

Changes

• monoprop_COMMPLACE: one COMMPLACE line per rank on stderr at propagator construction,
  reporting the launcher's actual CPU mask and masks=private/masks=shared; off by default,
  nothing computed when off.
• monoprop_PARTITION_PINNING deleted; pinning is now unconditional, and parse_env_flag now
  matches whole words instead of first characters.
• docs/features/parallelism.mdx swaps the row and states pinning is not configurable, with the
  reason.

Measurement

Neither piece can move a number: the diagnostic is report-only and off by default, and the deleted
knob was never set by any harness script. No campaign ran, and none is presented here.

Gates: ctest -L unit 222/222, -L serial 221/221, and 592 passed at each of four MPI layouts.

Relationship to the open PRs

Fully independent — zero overlapping files with either #259 or #263, so no stacking and no
merge interaction in any order. The refuted placement deletion it is salvaged from is unrelated to
the placement fix that merged as #249.

Checklist

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

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

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Docs preview: https://pr-269.monoprop-docs.pages.dev