Add Roslyn analyzer plugin

Bundle Microsoft.CodeAnalysis (C#) plus its dependencies under
Assets/Plugins/Roslyn for source-generator / analyzer support.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Assets/Plugins/Roslyn.meta

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+folderAsset: yes
+DefaultImporter:
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+  userData: 
+  assetBundleName: 
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Assets/Plugins/Roslyn/Microsoft.CodeAnalysis.CSharp.dll.meta

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Assets/Plugins/Roslyn/Microsoft.CodeAnalysis.dll.meta

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Assets/Plugins/Roslyn/System.Collections.Immutable.dll.meta

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Assets/Plugins/Roslyn/System.Reflection.Metadata.dll.meta

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Assets/Plugins/Roslyn/System.Runtime.CompilerServices.Unsafe.dll.meta

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