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

bindings/csharp/Directory.Packages.props

@@ -7,7 +7,7 @@
 
   <ItemGroup>
     <!-- Centralize Regorus package version with optional CI suffix -->
-    <PackageVersion Include="Regorus" Version="$(RegorusPackageVersion)$(RegorusPackageVersionSuffix)" />
+    <PackageVersion Include="Microsoft.Regorus" Version="$(RegorusPackageVersion)$(RegorusPackageVersionSuffix)" />
     <PackageVersion Include="MSTest" Version="3.8.2" />
     <PackageVersion Include="System.Text.Json" Version="8.0.5" />
     <PackageVersion Include="YamlDotNet" Version="13.7.0" />