Improve/api and packaging

[mar4elkin]

Summary

This PR adds NuGet packaging and for both .NET and Unity.

The repository now supports:

• a shared managed wrapper source without duplicated C# API files
• Unity distribution through annoy.net.unitypackage
• .NET distribution through the Annoy.Net NuGet package
• automated release publishing for native binaries, the Unity package, and the NuGet package

What changed

Shared managed source

• removed the duplicated annoy_sharp/Annoy.cs
• kept a single managed API source
• exposed the non-Unity wrapper through src/Annoy.Net/Annoy.Net.csproj

Unity packaging improvements

• relaxed Unity asmdef platform restrictions so the managed assembly is no longer Windows-only
• added Unity package samples under Samples~
• added Unity runtime tests under Tests/Runtime
• updated the Unity package metadata
• fixed .unitypackage generation so it imports cleanly into Assets/Annoy.NET
• ensured the release unitypackage includes native libraries for Windows, Linux, and macOS

NuGet packaging

• added package metadata for Annoy.Net
• added native runtime assets to the .nupkg for:
  • win-x64
  • linux-x64
  • osx-x64
• added a packaging script to build the NuGet artifact locally
• added a smoke test project that validates the package through a real PackageReference

CI and release pipeline

• extended GitHub Actions so release builds now:
• build and test native binaries on Windows, Linux, and macOS
• run managed smoke tests
• build annoy.net.unitypackage
• build Annoy.Net.nupkg
• attach native binaries, the Unity package, and the NuGet package to GitHub Releases
• added optional NuGet.org publishing on version tags when NUGET_API_KEY is configured

Validation

Validated locally with:

• native wrapper tests
• managed smoke tests against the native library
• local .unitypackage generation
• local .nupkg generation
• NuGet smoke test through a local package source and PackageReference

Notes

• the GitHub actions need to be tested; I'm not sure if they work

[not-heavychevy]

NuGet consumption is currently only tested on Linux, although package publishes native assets for win-x64, linux-x64 and osx-x64. There is a risk of missing the broken packaging path for Windows/macOS can safely reach the release unnoticed

[not-heavychevy]

collect .nupkg once, and then check its consumption on each target OS with a separate matrix-job. Now package-nuget and smoke test live only on ubuntu-24.04, although the package is assembled from Windows/Linux/macOS native artifacts, and .NET/NuGet choose native assets by runtimes/{ rid }/native/ for a specific runtime. Therefore, a successful Linux run does not validate win-x64 and osx-x64 boot paths.