windows: fix native MSVC/Vulkan build portability

[ElderOrb]

This PR makes the native Windows MSVC + Vulkan build path link and test cleanly enough to use the existing Windows CI lanes.

What changed:

• add a small cross-platform compatibility layer for POSIX-only file, env, pid, mmap, and aligned-allocation helpers
• switch Windows-facing codepaths and tests to those helpers instead of relying on POSIX APIs directly
• fix the Windows shared-library packaging/link path, including /WHOLEARCHIVE handling and explicit blake3_vendored linkage
• make the Vulkan loader and shared-library tests work on Windows without Unix-only dlopen assumptions
• keep MSVC warnings visible but stop promoting unrelated native-Windows warning cleanup into a hard build blocker
• gate or adapt tests whose harnesses are currently POSIX-only

Why:

• the upstream tree already has Windows CI lanes, but the native MSVC/Vulkan path still had several portability and packaging blockers
• this change is intended to make that path buildable and testable in GitHub Actions without changing the Linux behavior

Validation:

• the branch was validated on a native Windows Strix Halo host
• GitHub Actions should exercise the existing Windows jobs for this PR