Publish symbol packages

[chris-peterson]

Summary

• Add Source Link and symbol package (.snupkg) support for all NuGet packages
• Symbols are published to the nuget.org symbol server, enabling step-into debugging for consumers
• Remove redundant <DebugType>portable</DebugType> from csproj files (already the default)

How it works

As of .NET SDK 8, Source Link is built into the SDK — no package reference needed. The SDK automatically embeds source control metadata in PDBs, mapping source files to the exact GitHub commit.

The new Directory.Build.props configures:

• PublishRepositoryUrl — publishes the repo URL in NuGet package metadata (must be explicitly opted-in)
• IncludeSymbols / SymbolPackageFormat=snupkg — generates separate symbol packages (recommended over embedded PDBs to avoid ~30% package size bloat)
• ContinuousIntegrationBuild — normalizes file paths in PDBs on CI so they work across machines

What consumers get

Debugger (VS/Rider) automatically downloads PDBs from the nuget.org symbol server, then Source Link fetches the matching source from GitHub on demand.

References

• Source Link docs
• .NET library guidance: NuGet
• NuGet package authoring best practices

Test plan

• Verify snupkg files are generated alongside nupkg during build
• Verify publish workflow pushes both nupkg and snupkg to nuget.org
• Revert sourcelink branch trigger from publish.yml before merge