[WRONG BRANCH] test(config): preserve child readiness diagnostics

[luvs01]

Motivation

• A readiness helper consumed child.stderr on early exit, which prevented callers from reading the same single-consumption stream and masked the original diagnostics.

Description

• Stop consuming child.stderr in waitForOwnedChildReady, preserving the exit code and marker-path diagnostic without disturbing the stream.
• Add a regression test readiness failure leaves child stderr available to its caller that forces a child to exit and verifies the readiness error message and that callers can still read child.stderr.
• The change is confined to the test harness (tests/config/config-mutation-lock.test.ts) and preserves existing behavior for non-failure paths.

Testing

• Ran bun test tests/config/config-mutation-lock.test.ts --test-name-pattern 'readiness failure leaves child stderr available', which passed.
• Ran bun run typecheck and git diff --check, both of which succeeded.
• Ran the full file tests bun test tests/config/config-mutation-lock.test.ts; 27 tests passed and 1 test failed due to an unrelated environment limitation in Bun 1.2.14 (node:zlib zstdDecompressSync export), not caused by this change.

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Codex Task

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-10T02:35:30.313189Z	a475c6a	PR opened

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[github-actions]

✅ Deterministic PR hygiene checks passed.

[github-actions]

⏳ DRAFT

• wrong target branch (main); retarget to dev.

What to do

• Retarget this PR to dev — all contributions go to dev.

Its title has been prefixed with [WRONG BRANCH].
This pull request was already a draft. Its draft status will be preserved after every issue above is resolved.