Restore Fiberish.X86 native build detection so CI builds generate fibercpu before copy/link steps

[Copilot]

Recent CI runs regressed because Fiberish.X86 stopped detecting the in-repo libfibercpu source tree during project evaluation. As a result, MSBuild skipped the native CMake target and later failed when packaging/linking fibercpu outputs across Linux, macOS, Windows, and browser-wasm builds.

• What changed

  • Moved LibFibercpuSourceDir and LibFibercpuSourceExists back into the top-level PropertyGroup in Fiberish.X86.csproj.
  • This restores evaluation-time availability for:
    • BuildNativeLib target conditions
    • incremental native build inputs
    • output copy/link items that depend on generated native artifacts

• Why this matters

  • The incremental-build refactor left source detection scoped too narrowly, so BuildNativeLib could be skipped before MSBuild knew the native source tree existed.
  • Restoring these properties at project scope ensures CMake runs before managed build steps attempt to copy libfibercpu.so, libfibercpu.dylib, fibercpu.dll, or libfibercpu.a.

• Effect on build paths

  • Unblocks dynamic library copy on host/native targets.
  • Unblocks static archive generation for browser-wasm.
  • Preserves the existing RID-specific native output layout and incremental tracking behavior.

• Representative change

<PropertyGroup>
  <LibFibercpuSourceDir>$(MSBuildThisFileDirectory)../libfibercpu</LibFibercpuSourceDir>
  <LibFibercpuSourceExists Condition="Exists('$(LibFibercpuSourceDir)/CMakeLists.txt')">true</LibFibercpuSourceExists>
  <LibFibercpuSourceExists Condition="'$(LibFibercpuSourceExists)' == ''">false</LibFibercpuSourceExists>
</PropertyGroup>