Fix MSVC mutex initialization with older host runtimes

[zachlewis]

OCIO built with recent MSVC toolchains may crash when a host application supplies an older MSVC runtime. The newer constexpr mutex constructor skips the runtime initialization that older runtimes expect before locking. This is suspected to cause the OCIO initialization crash reported in OIIO #5493 and discussed on OIIO #5504.

Define _DISABLE_CONSTEXPR_MUTEX_CONSTRUCTOR in OCIO's shared MSVC compiler settings, following OIIO #4914, which addressed OIIO #4641. This restores runtime mutex initialization throughout the build, including the internal Mutex wrapper and direct std::mutex uses.

Validation:

• CMake checks confirm the definition is enabled for MSVC and absent for Clang.
• AppleClang Release build of the library and CPU/utility tests passed.
• All four CTest targets passed: utility tests and CPU tests with default, no-acceleration, and SSE2 settings. The initial sandboxed run could not discover system monitors; rerunning outside the sandbox passed.
• git diff --check passed.

The Windows/Nuke reproduction and compatibility with an older MSVC runtime have not been tested locally.

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Assisted-by: OpenAI Codex (GPT-6 Sol)

[cozdas]

LGTM!

[carolalynn]

This is great please just disclose ai tool use with “assisted by: tool / model” in the pr body if you used one :)

[zachlewis]

Oh lordy, I didn't realize I hadn't done that. Shame on me!