Add gated cross-platform CI using preinstalled compilers

[wilx]

Automate ThreadPool's regression tests on Linux, Windows, and macOS using only preinstalled runner toolchains and one Release configuration. GCC 12/C++11 runs first and gates every other compiler job; each compiler then tests its requested language modes in separate build directories.

Add a standalone CMake/CTest build and 14 portable behavior cases alongside the 31 Linux failure-injection cases, with a 90-second timeout per case. Update the test documentation. An internal result trait also fixes VS 2015's variadic alias expansion while preserving the public API.

Validation:

• All 13 hosted CI jobs passed.
• Local GCC 12/13/14 and Clang 18 builds passed in C++11, 17, 20, and 23.
• Workflow lint, PowerShell syntax, and CTest registration checks passed.
• A deliberately failed C++11 gate skipped every dependent job.
• The VS 2015 result-trait fix passed the portable suite in a hosted v140 job.

[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 2026-10-06T20:06:31.608908Z	125738d	PR opened

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