fix: detect winding direction for correct part spacing offset

PolygonHelper.ExtractPerimeterPolygon always used OffsetSide.Right
assuming CCW winding, but DXF imports can produce CW winding. This
caused the spacing polygon to shrink inward instead of expanding
outward, making parts overlap during nesting.

Now detects winding direction via polygon signed area and selects
the correct OffsetSide accordingly.

Also adds save_nest MCP tool and a BOM-to-nest builder utility
(tools/NestBuilder) for batch-creating nest files from Excel BOMs.

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

tools/NestBuilder/NestBuilder.csproj

@@ -0,0 +1,10 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net8.0-windows</TargetFramework>
+  </PropertyGroup>
+  <ItemGroup>
+    <ProjectReference Include="..\..\OpenNest.Core\OpenNest.Core.csproj" />
+    <ProjectReference Include="..\..\OpenNest.IO\OpenNest.IO.csproj" />
+  </ItemGroup>
+</Project>