[akhilkalyani]

Port the Unity package (MCPForUnity) so it compiles and runs on Unity 2020.3 (C# 8, .NET 4.x profile). The Python server is unchanged (mcpforunityserver==10.3.0).

• Rewrite C# 9 syntax to C# 8: target-typed new(), is not patterns, or/and patterns, target-typed conditionals, ranges.
• Replace .NET Standard 2.1 APIs missing from 2020.3 with Editor/Compat/NetCompat.cs (string overloads, Math.Clamp, Path.GetRelativePath, ProcessStartInfo.ArgumentList, PBKDF2-SHA256).
• Guard Unity 2021+ editor APIs behind UNITY_2021_2/2021_3_OR_NEWER with 2020.3 fallbacks; add stand-ins for DropdownField, NamedBuildTarget, StandaloneBuildSubtarget and PackageInfo.GetAllRegisteredPackages.
• Runtime UI Toolkit actions of manage_ui return a clear error on 2020.3.
• Drop the com.unity.nuget.newtonsoft-json dependency (conflicts with jillejr Newtonsoft packages), reference the TestRunner assemblies explicitly, and disable upstream update prompts.
• Add UNITY-2020.3.md and tools/unity-2020.3-backport (compile check, C# 9->8 rewriter, tarball packer).

Description

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactoring (no functional changes)
• Test update

Changes Made

Compatibility / Package Source

• Unity version(s) tested:
• Package source used (#beta, #main, tag, branch, or file:):
• Resolved commit hash from Packages/packages-lock.json (if using a Git package URL):

Testing/Screenshots/Recordings

• Python tests (cd Server && uv run pytest tests/ -v)
• Unity EditMode tests
• Unity PlayMode tests
• Package import/compile check
• Not applicable (explain why in Additional Notes)

Documentation Updates

• I have added/removed/modified tools or resources
• If yes, I have updated all documentation files using:
  • The LLM prompt at tools/UPDATE_DOCS_PROMPT.md (recommended)
  • Manual review of the generated changes

Related Issues

Additional Notes

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