Build failure: CS8795 with dotnet 9.x - analyzer version mismatch

[M-r-A]

The build fails on all .NET 9.0.x SDKs with CS8795 (partial method without implementation) due to an analyzer version incompatibility introduced in commit 075ab81.

Summary

SourceGen.csproj depends on Microsoft.CodeAnalysis.CSharp 5.3.0, which requires Roslyn 5.3.0+. However:

• .NET SDK 9.0.x bundles Roslyn 4.14.0 ❌ (too old)
• .NET SDK 10.0.302+ bundles Roslyn 5.3.0+ ✓ (compatible)

The build workflow specifies dotnet-version: 9.x, so it triggers the mismatch locally for anyone building on 9.0.x.

Steps to Reproduce

# On any .NET 9.0.x SDK (e.g., 9.0.315, 9.0.316):
dotnet build Source/Multiplayer.sln

Expected Behavior

Build succeeds with warnings only.

Actual Behavior

CSC : warning CS9057: Analyzer assembly 'SourceGen.dll' cannot be used because 
      it references version '5.3.0.0' of the compiler, which is newer than the 
      currently running version '4.14.0.0'.

D:/.../ChatCommandRegistry.cs(18,32): error CS8795: Partial method 
  'ChatCommandRegistry.Register(ChatCommandManager, MultiplayerServer)' 
  must have an implementation part because it has accessibility modifiers.

Build FAILED.