Make Windows Vulkan support optional

[neguse]

Summary

• detect the Vulkan SDK quietly on Windows and omit the Vulkan backend when it is unavailable
• keep Vulkan required for Linux, where it remains the default native backend
• guard runtime Vulkan references and document the Windows SDK as optional

Validation

• fresh Windows Release configure and build without a Vulkan SDK: pwsh -NoProfile -ExecutionPolicy Bypass -File ./scripts/build-release.ps1 -BuildDir build-release-no-vulkan
• scripts/pre-commit.sh
• confirmed the generated Ninja rules omit backend_vulkan.c and Vulkan::Vulkan