feat: add embedded CL-980 default config with first-run EnsureDefaults

Embeds CL-980.json as a resource in OpenNest.Data and adds EnsureDefaults()
to LocalJsonProvider, which seeds the machines directory on first run when empty.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

OpenNest.Data/OpenNest.Data.csproj

@@ -9,4 +9,7 @@
   <ItemGroup>
     <ProjectReference Include="..\OpenNest.Core\OpenNest.Core.csproj" />
   </ItemGroup>
+  <ItemGroup>
+    <EmbeddedResource Include="Defaults\CL-980.json" />
+  </ItemGroup>
 </Project>