refactor: replace SDK-style project references with direct imports

Projects no longer use `<Project Sdk="Stride.Build.Sdk">`. Every .csproj
now imports SDK props/targets directly from source:

  <Project>
    <Import Project="$([MSBuild]::GetDirectoryNameOfFileAbove(...))\sdk\...\Sdk.props" />
    ...
    <Import Project="$(StrideRoot)sources\sdk\...\Sdk.targets" />
  </Project>

The props import uses GetDirectoryNameOfFileAbove (no pre-set property
needed) and the targets import uses $(StrideRoot) (safe after the SDK
chain is loaded).

SDK internal cross-references (Editor and Tests SDKs) now use
$(MSBuildThisFileDirectory) relative paths instead of the Sdk= resolver.

Other changes:
- global.json: msbuild-sdks entries commented out
- nuget.config: stride-sdks source and packageSourceMapping commented out
- .github/actions/build-sdk: removed (no longer a prerequisite)
- build/compile.bat: removed SDK build step
- sources/Directory.Build.props: replaced TODO with explanatory comment
- build/docs/SDK-GUIDE.md: added direct-import section and revert guide
- sources/sdk/README.md: new file explaining import modes and revert steps

To revert to full-SDK mode: build SDK packages, uncomment global.json
and nuget.config entries, restore <Project Sdk="..."> opening tags, and
revert the four SDK internal files. See sources/sdk/README.md.

Author: Kryptos-FR

.github/actions/build-sdk/action.yml

@@ -76,7 +76,6 @@ jobs:
           echo "ANDROID_NDK_ROOT=$ndkPath" >> $env:GITHUB_ENV
           echo "ANDROID_NDK_HOME=$ndkPath" >> $env:GITHUB_ENV
           echo "Using NDK at: $ndkPath"
-      - uses: ./.github/actions/build-sdk
       - name: Debug NDK Configuration
         shell: pwsh
         run: |

.github/workflows/build-android.yml

@@ -48,7 +48,6 @@ jobs:
       - uses: actions/setup-dotnet@v4
         with:
           dotnet-version: '10.0.x'
-      - uses: ./.github/actions/build-sdk
       - name: Build
         run: |
           dotnet build build\Stride.AssemblyProcessor.sln `

.github/workflows/build-assembly-processor.yml

@@ -54,7 +54,6 @@ jobs:
           dotnet-version: '10.0.x'
       - name: Install .NET iOS Workload
         run: dotnet workload install ios
-      - uses: ./.github/actions/build-sdk
       - name: Build
         run: |
           dotnet build build\Stride.iOS.slnf `

.github/workflows/build-ios.yml

@@ -50,7 +50,6 @@ jobs:
       - uses: actions/setup-dotnet@v4
         with:
           dotnet-version: '10.0.x'
-      - uses: ./.github/actions/build-sdk
       - name: Build
         run: |
           dotnet build build\Stride.Launcher.sln `

.github/workflows/build-launcher.yml

@@ -62,7 +62,6 @@ jobs:
       - uses: actions/setup-dotnet@v4
         with:
           dotnet-version: '10.0.x'
-      - uses: ./.github/actions/build-sdk
       - name: Build
         run: |
           dotnet build build\Stride.Runtime.slnf `

.github/workflows/build-linux-runtime.yml

@@ -50,7 +50,6 @@ jobs:
       - uses: microsoft/setup-msbuild@v3
         with:
           vs-version: '[18.0,19.0)'
-      - uses: ./.github/actions/build-sdk
       - name: Build
         run: |
           msbuild build\Stride.VisualStudio.slnx `

.github/workflows/build-vs-package.yml

@@ -47,7 +47,6 @@ jobs:
       - uses: actions/setup-dotnet@v4
         with:
           dotnet-version: '10.0.x'
-      - uses: ./.github/actions/build-sdk
       - name: Build
         run: |
           dotnet build build\Stride.slnx `

.github/workflows/build-windows-full.yml

@@ -65,7 +65,6 @@ jobs:
       - uses: actions/setup-dotnet@v4
         with:
           dotnet-version: '10.0.x'
-      - uses: ./.github/actions/build-sdk
       - name: Build
         run: |
           dotnet build build\Stride.Runtime.slnf `

.github/workflows/build-windows-runtime.yml

@@ -49,7 +49,6 @@ jobs:
         run: |
           sudo apt-get update
           sudo apt-get install -y libbsd-dev clang llvm lld
-      - uses: ./.github/actions/build-sdk
       - name: Patch NativePath for Linux
         run: |
           # Make strlcat_chk and strlcpy_chk non-fatal on Linux