Add cargo ci dlls command for building C# DLLs and NuGet packages

[rekhoff]

Description of Changes

• Added a new cargo ci dlls subcommand to build/pack the in-repo C# NuGet packages and the C# SDK.
• cargo ci dlls restores sdks/csharp/SpacetimeDB.ClientSDK.csproj using the freshly built local package outputs as to populate sdks/csharp/packages/**.
• Added a Unity .meta skeleton under sdks/csharp/unity-meta-skeleton~/** and overlays those .meta files onto the latest restored versioned package directory to keep Unity GUIDs stable and import settings consistent.
• Unity-specific import fixes are captured in the skeleton overlay (notably: preventing Unity from importing incompatible TFMs like net8.0, and marking analyzer DLLs with the RoslynAnalyzer label so Unity can recognize them).

How to use (local)

# Build/pack + restore local packages into sdks/csharp/packages/**
cargo ci dlls

API and ABI breaking changes

N/A

Expected complexity level and risk

2 - Local developer tooling + file overlay into restore output; no runtime/SDK API behavior changes.

Testing

• cargo check -p ci
• Ran cargo ci dlls and verified the output under sdks/csharp/packages/** and the various NuGet package locations.
• Tested a Unity project importing the SpacetimeDB SDK after generating output and confirmed no errors.

[bfops]

This looks pretty great to me. I left some nits, and I think there may be some remaining meta dance to do.

also, I see more files getting restored such as LICENSE and logo.png, so I think we should possibly do what #3648 did and add a gitignore for those files as well.

[rekhoff]

This looks pretty great to me. I left some nits, and I think there may be some remaining meta dance to do.

also, I see more files getting restored such as LICENSE and logo.png, so I think we should possibly do what #3648 did and add a gitignore for those files as well.

Updated .gitignore to ignore those restore artifacts and keep only lib/** + analyzers/**

[jdetter]

I did a quick skim of this - I don't have anything blocking. Thanks Ryan 👍

I'm not approving as Zeke has the primary review on this, but if more review from me is needed let me know 🙂

[bfops]

I thought I had already approved. LGTM! Thanks @rekhoff .

Someday it would be nice to add tests for some of the behavior, like the skeleton directory overlaying, or that dotnet restore has meta files on all platforms, but that seems out of scope for now since they're pretty easy to hand-test.