feat: auto-copy Cincinnati post processor DLL to Posts/ on build

The WinForms app's LoadPosts() scans Posts/ for IPostProcessor DLLs.
A build target copies the Cincinnati DLL there so it appears in the
Nest > Post menu automatically.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest.Posts.Cincinnati/OpenNest.Posts.Cincinnati.csproj

@@ -6,4 +6,11 @@
   <ItemGroup>
     <ProjectReference Include="..\OpenNest.Core\OpenNest.Core.csproj" />
   </ItemGroup>
+  <Target Name="CopyToPostsDir" AfterTargets="Build">
+    <PropertyGroup>
+      <PostsDir>..\OpenNest\bin\$(Configuration)\$(TargetFramework)\Posts\</PostsDir>
+    </PropertyGroup>
+    <MakeDir Directories="$(PostsDir)" />
+    <Copy SourceFiles="$(TargetPath)" DestinationFolder="$(PostsDir)" SkipUnchangedFiles="true" />
+  </Target>
 </Project>