build(csharp): prepare NuGet package for nuget.org publishing (#668)

* build(csharp): rename NuGet package to Microsoft.Regorus

Align the NuGet package identity with the Microsoft.Regorus
namespace and the Microsoft.* reserved prefix on nuget.org
in preparation for publishing the package.

- Add explicit <PackageId>Microsoft.Regorus</PackageId>
- Update PackageVersion in Directory.Packages.props
- Update version bump regex for new package name

* build(csharp): default to ProjectReference for in-repo consumers

Use ProjectReference for Regorus.Tests, Benchmarks, TestApp,
and TargetExampleApp so that local development does not require
a pre-built .nupkg. PackageReference mode remains available via
/p:UsePackageReference=true for validating the packaged NuGet.

* build(csharp): add nuget.config with source mapping and xtask integration

Add explicit NuGet configuration to ensure in-repo builds always
resolve Microsoft.Regorus from the locally built package, even
after the package is published on nuget.org.

- Add nuget.config with <clear/> and packageSourceMapping
- Update xtask to copy .nupkg into local-packages/ directory
- Pass /p:UsePackageReference=true from xtask for CI testing
- Add restore validation step to the xtask test flow
- Add .gitignore for the local-packages directory

.github/workflows/test-csharp.yml

@@ -109,8 +109,8 @@ jobs:
         with:
           name: regorus-nuget
           path: |
-            bindings/csharp/Regorus/bin/Release/Regorus*.nupkg
-            bindings/csharp/Regorus/bin/Release/Regorus*.snupkg
+            bindings/csharp/Regorus/bin/Release/Microsoft.Regorus*.nupkg
+            bindings/csharp/Regorus/bin/Release/Microsoft.Regorus*.snupkg
           if-no-files-found: error
           retention-days: 1