Fix strict builds with current toolchains (#99)

* Suppress intentional Windows struct padding warnings

🧙 Conjured by AI via [pi.dev](https://pi.dev/) using gpt-5.6

* Update vendored utest.h

Includes the intentional global constructor warning fix from sheredom/utest.h#184.

🧙 Conjured by AI via [pi.dev](https://pi.dev/) using gpt-5.6

* Support the standardized macOS spawn chdir action

Use posix_spawn_file_actions_addchdir for macOS 26 deployment targets and suppress the intentional deprecation warning when retaining the backward-compatible _np API.

🧙 Conjured by AI via [pi.dev](https://pi.dev/) using gpt-5.6

* Avoid wchar_t C++ keyword compatibility warnings

Use an internal wide-character alias so strict clang-cl C builds only need to suppress the compatibility warning at the intentional wchar_t typedef.

🧙 Conjured by AI via [pi.dev](https://pi.dev/) using gpt-5.6

* Suppress intentional TLS object duplication warning

The named-pipe counter is intentionally local to each header-library instance.

🧙 Conjured by AI via [pi.dev](https://pi.dev/) using gpt-5.6

* Ignore Windows SDK enum migration warnings

MSVC 2026 emits C4865 from Windows SDK headers under /Wall; suppress that external-header warning in the test build.

🧙 Conjured by AI via [pi.dev](https://pi.dev/) using gpt-5.6