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/Regorus.Tests/Regorus.Tests.csproj

@@ -12,6 +12,7 @@
   <PropertyGroup>
     <!-- If the environment variable is set (such as in a Github Action run), append the suffix to the version number -->
     <RegorusPackageVersionSuffix Condition="'$(VersionSuffix)' != ''">-$(VersionSuffix)</RegorusPackageVersionSuffix>
+    <UsePackageReference Condition="'$(UsePackageReference)' == ''">false</UsePackageReference>
   </PropertyGroup>
 
   <ItemGroup>
@@ -23,8 +24,12 @@
     <PackageReference Include="YamlDotNet" />
   </ItemGroup>
 
-  <ItemGroup>
-    <PackageReference Include="Regorus" />
+  <ItemGroup Condition="'$(UsePackageReference)' != 'true'">
+    <ProjectReference Include="../Regorus/Regorus.csproj" />
+  </ItemGroup>
+
+  <ItemGroup Condition="'$(UsePackageReference)' == 'true'">
+    <PackageReference Include="Microsoft.Regorus" />
   </ItemGroup>
 
   <ItemGroup>