test(helper,docs): the parallel worker default follows the machine, not a flat 4

[dinndev2]

What

The suite's parallel worker default was a flat NCPU=4. It now follows the machine:
Wurk::Test::DEFAULT_NCPU = (Etc.nprocessors / 2).clamp(1, 4). A 4-core box forks 2
workers, an 8-core one still forks 4, and an explicit NCPU=<n> is untouched.

Why

test_helper's own comment has always justified the default as "deliberately below the
core count of most machines that run this", and the same paragraph warns that one worker
per core "oversubscribes badly, and the failures it produces are wall-clock ones (a drain
that doesn't finish, a socket read that times out) rather than honest assertion failures".

A flat 4 stopped honouring that the moment the suite ran on a 4-core machine, where 4
workers IS one worker per core. Two places that happens today:

• The developerz.ai fleet boxes — 4 cores at 1.80 GHz, several sessions at once —
  run ./bin/check on an unmodified checkout as a merge gate. Over the 30 days to
  2026-09-18 that gate refused 12 tasks with 103 box_baseline_red events, and the test
  methods extracted from those tails were SwarmSupervisionTest#test_kill_replacement_mid_restart
  (83), LimiterStressTest#test_1000_concurrent_acquires_never_exceed_the_limit (3) and
  SwarmSupervisionTest#test_crash_loop_backoff_grows_and_term_drains_while_pending (2) —
  every one of them timing-sensitive, and every one fixed by test(swarm,limiter): the waits follow progress, and the stress test asserts atomicity rather than the host's clock #535.
• CI's own fallback runner, ubuntu-latest, is 4 vCPU.

#535 fixed the three tests. This removes the remaining way to reproduce the class without
an operator remembering to set NCPU=2 in a box's environment — a lever that has to be
re-applied on every machine and survives no reprovision.

Changes

• Wurk::Test::DEFAULT_NCPU beside the other parallelism constants; the ENV['NCPU']
  line reads it instead of the literal 4. The explicit-override path and the
  clamp(1, WORKER_DATABASES) Redis-isolation ceiling are unchanged.
• The NCPU comment block now states the rule it implements, and carries the measurement
  below so the next reader does not have to re-derive the cost.
• test/unit/ncpu_default_test.rb — four assertions on the constant's PROPERTIES, not its
  formula: never more workers than cores, never above 4, never below 1, never above
  WORKER_DATABASES.
• CLAUDE.md's CI standard, which documented "workers stay at NCPU=4 … which is why the
  suite runner wants roughly a 4 vCPU box". Left alone it would assert a default the code
  no longer has.

Verification

Measured against main (308bdd7) on a 6-core host, with taskset -c 0-3 giving the suite
a 4-core view — Etc.nprocessors honours CPU affinity, so this is the same arithmetic a
4-core box does:

run	result	wall clock
NCPU=4, 4 cores	4316 runs, 11915 assertions, 0 failures, 6 skips	4m57s
NCPU=2, 4 cores	4316 runs, 11915 assertions, 0 failures, 6 skips	5m24s

So halving the workers costs 9% of the gate, not double — this suite waits far more
than it computes.

Stated plainly: this is not a local reproduction of the box failure. Main's tip is
green at 4 workers on 4 cores. The fleet boxes differ in two ways taskset cannot
reproduce — a 1.80 GHz clock and several sessions sharing the box — so this change is
about the machine the suite is given, not about a red I could produce here.

Also checked:

• The default resolves to 3 on the 6-core host and 2 under taskset -c 0-3; an explicit
  NCPU=7 is still honoured.
• The new test was run red first: with the constant deliberately set to
  Etc.nprocessors * 2 it fails with Expected 12 to be <= 6.
• bundle exec rubocop clean on both changed Ruby files.
• ./bin/check green with the patch applied, branch off origin/main at 308bdd7
  (the default resolves to 3 on this 6-core host): 4318 runs, 11923 assertions,
  0 failures, 0 errors, 6 skips, parity oracles 23/23, exit 0, 3m17s. The 4318 is the
  4316 of the table plus this PR's two.

Refs developerz-ai/developerz.ai#4386, #522.

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Summary by CodeRabbit

• Tests

  • Test execution now defaults to using half of the available CPU cores, with a minimum of 1 and a maximum of 4 workers.
  • Explicit worker-count overrides remain supported.
  • Added coverage to validate safe worker limits across different machine configurations.

• Documentation

  • Updated CI guidance and timing examples to reflect the new adaptive default.

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📝 Walkthrough

Walkthrough

The test environment now derives its default worker count from available processors. The value is bounded from 1 to 4. Explicit NCPU overrides remain supported and bounded by the available worker databases.

Changes

Adaptive worker defaults

Layer / File(s)	Summary
Worker default and environment wiring test/test_helper.rb	The test helper uses Etc.nprocessors to calculate Wurk::Test::DEFAULT_NCPU as half the processor count, clamped from 1 to 4. The exported NCPU value retains explicit overrides and clamps values to the available worker databases.
Documentation and default validation CLAUDE.md, test/unit/ncpu_default_test.rb	CI documentation describes the adaptive default. Tests verify scaling across processor counts and confirm the machine-level default stays within the worker database limit.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix

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Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: the parallel worker default now adapts to the machine instead of using a fixed value of 4.
Docstring Coverage	✅ Passed	Docstring coverage is 33.33% which is sufficient. The required threshold is 30.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 3 functions across 2 files.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Inline comments:
In `@test/unit/ncpu_default_test.rb`:
- Line 18: Update the DEFAULT_NCPU test to assert equality with the adaptive
calculation (Etc.nprocessors / 2).clamp(1, 4), rather than only checking an
upper bound. If needed to test varying processor counts, expose the calculation
through a callable method and assert its result across processor counts.

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[dinndev2]

Taken in substance, not in form.

The literal suggestion — assert DEFAULT_NCPU == (Etc.nprocessors / 2).clamp(1, 4) — re-derives the expression the constant already holds, so it passes for whatever arithmetic is inside it and fails only if the two copies drift. That is a change-detector, not a test.

The gap behind the comment is real, though: a bounds-only test also passes if the default is hard-wired to 1 on every machine. So I took the second half of your suggestion and made the rule callable:

WORKER_CAP = 4
def self.default_ncpu(cores) = (cores / 2).clamp(1, WORKER_CAP)
DEFAULT_NCPU = default_ncpu(Etc.nprocessors)

and the test now states what the rule does on machines this one is not:

1 -> 1   2 -> 1   3 -> 1   4 -> 2   6 -> 3   8 -> 4   12 -> 4   64 -> 4

4 is the fleet box this PR is about, 6 a developer laptop, 64 a CI runner. One case still pins DEFAULT_NCPU to the rule on the running machine, so the constant every run uses is covered too.

Verified red before green: with the rule changed to one worker per core, the table fails. ./bin/check after: 4319 runs, 11935 assertions, 0 failures, exit 0.

[developerz-ai]

Review summary — 3 file(s), 0 finding(s).

PR #536 makes the parallel-worker default follow the machine and adds a WORKER_DATABASES cap assertion, but the diff excerpt provided is too truncated (a few closing end lines plus one partial assert_operator) to anchor a concrete, verifiable defect.

No findings from: concern-security, concern-tests.

(Some reviewers completed only 2 of 3 review samples; findings are the union of the samples that completed.)

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