build(mcp): target net8.0 and pin package versions

The MCP server uses no Windows APIs, so net8.0-windows only kept it from
building on Linux. Floating 0.*-* / 9.* versions let a restore silently pick
up a new ModelContextProtocol preview; pin the currently resolved versions.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

OpenNest.Mcp/OpenNest.Mcp.csproj

@@ -2,7 +2,7 @@
 
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFramework>net8.0-windows</TargetFramework>
+    <TargetFramework>net8.0</TargetFramework>
     <RootNamespace>OpenNest.Mcp</RootNamespace>
     <AssemblyName>OpenNest.Mcp</AssemblyName>
   </PropertyGroup>
@@ -11,8 +11,8 @@
     <ProjectReference Include="..\OpenNest.Core\OpenNest.Core.csproj" />
     <ProjectReference Include="..\OpenNest.Engine\OpenNest.Engine.csproj" />
     <ProjectReference Include="..\OpenNest.IO\OpenNest.IO.csproj" />
-    <PackageReference Include="ModelContextProtocol" Version="0.*-*" />
-    <PackageReference Include="Microsoft.Extensions.Hosting" Version="9.*" />
+    <PackageReference Include="ModelContextProtocol" Version="0.9.0-preview.2" />
+    <PackageReference Include="Microsoft.Extensions.Hosting" Version="9.0.21" />
   </ItemGroup>
 
 </Project>