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>

OpenNest.Mcp/Tools/InputTools.cs

@@ -57,6 +57,35 @@ namespace OpenNest.Mcp.Tools
             return sb.ToString();
         }
 
+        [McpServerTool(Name = "save_nest")]
+        [Description("Save the current session (all drawings and plates) to a .nest file.")]
+        public string SaveNest(
+            [Description("Absolute path for the output .nest file")] string path,
+            [Description("Name for the nest (optional)")] string name = null)
+        {
+            var nest = new Nest();
+            nest.Name = name ?? Path.GetFileNameWithoutExtension(path);
+
+            foreach (var drawing in _session.AllDrawings())
+                nest.Drawings.Add(drawing);
+
+            foreach (var plate in _session.AllPlates())
+                nest.Plates.Add(plate);
+
+            if (nest.Drawings.Count == 0)
+                return "Error: no drawings in session to save";
+
+            var dir = Path.GetDirectoryName(path);
+            if (!string.IsNullOrEmpty(dir) && !Directory.Exists(dir))
+                Directory.CreateDirectory(dir);
+
+            var writer = new NestWriter(nest);
+            if (!writer.Write(path))
+                return "Error: failed to write nest file";
+
+            return $"Saved nest to {path}\n  Drawings: {nest.Drawings.Count}\n  Plates: {nest.Plates.Count}";
+        }
+
         [McpServerTool(Name = "import_dxf")]
         [Description("Import a DXF file as a new drawing. Returns drawing name and bounding box.")]
         public string ImportDxf(