[codex] stabilize stress descendant test

[greenthree]

Summary

• atomically publish the stress checker child PID used by the descendant cleanup test
• keep the real timeout and process-tree death assertions unchanged

Root cause

Main CI run 31511756536 failed only on Windows / Python 3.10 because Path.write_text briefly exposed an empty PID file before writing the PID. The checker timeout could terminate the child during that window, leaving the test to parse "" as an integer. The production process cleanup completed; the failure was in the test handshake and is unrelated to the typeid(type-id) mutation change.

Validation

• focused descendant cleanup test: 30/30 consecutive passes
• python -m unittest tests.test_stress -v: 15 passed
• npm run check: 713 passed, 30 skipped
• npm run pack:check: probhub 223 files, probhub-skill 5 files
• local phtest nine-problem lint: ok
• git diff --check: passed