Tooling/clang-cl cross build

[Belonit]

Summary

Add an experimental Linux-hosted Win32 cross-build using clang-cl, LLVM tools,
LLD, and UASM with the MSVC ABI, headers, and libraries. The work also makes
the source and legacy assembly accepted by clang-cl, and adds a VS Code
IntelliSense example for the configuration.

Behavior and compatibility

Behavior preserved for supported Visual Studio 2022 Win32 Debug and Release
builds. The clang-cl configuration is explicitly unsupported and intended for
compiler-portability work; it does not expand the supported build matrix.

The cross-build does not change game data formats, saves, replays, networking,
deterministic simulation, COM interfaces, or externally consumed ABI. The
existing codebase may contain undefined behavior that is not exposed by the
supported MSVC build, so a successful clang-cl build is not runtime evidence.

Known clang-cl runtime observations:

• Without fix: UB in movement path advancement #76, existing
  undefined behavior causes the game to crash.
• Without fix: voxel cache rendering corruption #77), existing
  undefined behavior produces rendering artifacts on voxels.

Validation

cmake -S . -B build/clang-cl -G Ninja \
  -DCMAKE_BUILD_TYPE=Release \
  -DCMAKE_TOOLCHAIN_FILE=cmake/toolchains/clang-cl-msvc.cmake \
  -DOPENTS_MSVC_ROOT=<msvc-root>
cmake --build build/clang-cl

Configured and built with clang-cl 22.1.8, MSVC 14.44.35207, and Windows SDK
10.0.26100.0. The build completed with inherited warnings and no errors.

wine build/clang-cl/bin/LogStress.exe

All logger checks passed, including 100,000 intact log lines.

Supported Visual Studio 2022 Win32 Debug and Release builds were not run before
submitting this PR; CI remains the required validation for the supported matrix.

Documentation

Updated docs/BUILDING.md with the experimental toolchain requirements,
configuration command, and the runtime limitation caused by potentially
unexposed undefined behavior.

Checklist

• The change is focused; unrelated mechanical cleanup is separate
• Compatibility effects and any migration are explicit
• Validation distinguishes what passed, failed, and was not run
• No prohibited assets, binaries, SDKs, credentials, or generated output are included