fix: include Reentry.Core in slnx for Visual Studio restore

[piranout]

Follow-up to #2 (already merged). The x64 pin (and a local <Platform Project="x64" /> on App) cleared the unknown-configuration banner. Rebuild then failed:

NU1105: Unable to find project information for '...\\src\\Reentry.Core\\Reentry.Core.csproj'.
If you are using Visual Studio, this may be because the project is unloaded or not part of the current solution

Reentry.Core is a ProjectReference from App but was not in the slnx.

Change

• Add src/Reentry.Core/Reentry.Core.csproj under /src/.
• Keep a single solution platform x64 (no Any CPU).
• Map both projects to x64.
• Core declares <Platforms>x64</Platforms> so the slnx mapping is valid. TFM stays net10.0 (not -windows).
• App still lists x86;x64;ARM64. Tests stay out of the slnx.

<Solution>
  <Configurations>
    <Platform Name="x64" />
  </Configurations>
  <Folder Name="/src/">
    <Project Path="src/Reentry.Core/Reentry.Core.csproj">
      <Platform Project="x64" />
    </Project>
    <Project Path="src/Reentry.App/Reentry.App.csproj">
      <Platform Project="x64" />
    </Project>
  </Folder>
</Solution>

Verify

• slnx has Core + App, x64 only, no Any CPU
• Linux dotnet test tests/Reentry.Core.Tests — 39 passed
• CI dotnet build Reentry.slnx -c Release
• Visual Studio restore/rebuild builds Reentry.App