Merge Slang compiler projects

[botoddly]

Summary

• merge GameKit.SdlangCompileLib and GameKit.SdlangCompileTask into GameKit.SdlangCompiler
• combine compiler and task tests under GameKit.SdlangCompiler.Tests
• consolidate Slang configuration, download targets, task registration, and shader compilation targets
• update all project references, imports, integration fixtures, tutorials, and documentation
• retain SdlangCompileTask as the MSBuild task name while removing the old assemblies and namespaces

Verification

• dotnet build GameKit.slnx -p:TreatWarningsAsErrors=true
• dotnet test GameKit.slnx --no-build -p:TreatWarningsAsErrors=true
• dotnet test tests/GameKit.SdlangBuildIntegration.Tests/GameKit.SdlangBuildIntegration.Tests.csproj --no-build --filter 'TestCategory=BuildIntegration' -p:TreatWarningsAsErrors=true
• dotnet test tests/GameKit.Logging.Tests/GameKit.Logging.Tests.csproj --configuration Release -p:TreatWarningsAsErrors=true
• dotnet publish tutorials/GameKit.Tutorials.Logging/GameKit.Tutorials.Logging.csproj --configuration Release --runtime linux-x64 -p:TreatWarningsAsErrors=true

The repository's linux-arm64 NativeAOT command reached native linking locally but requires the ARM64 linker available on the ARM CI runner.