feat(io): add NestWriter.Write(Stream) overload

Adds a Write(Stream) overload that writes the ZIP archive to any stream
with leaveOpen: true so the caller can read back a MemoryStream after
the ZipArchive is disposed. Refactors Write(string) to delegate to the
new overload.

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

OpenNest.IO/NestWriter.cs

@@ -28,12 +28,17 @@ namespace OpenNest.IO
         }
 
         public bool Write(string file)
+        {
+            using var fileStream = new FileStream(file, FileMode.Create);
+            return Write(fileStream);
+        }
+
+        public bool Write(Stream stream)
         {
             nest.DateLastModified = DateTime.Now;
             SetDrawingIds();
 
-            using var fileStream = new FileStream(file, FileMode.Create);
-            using var zipArchive = new ZipArchive(fileStream, ZipArchiveMode.Create);
+            using var zipArchive = new ZipArchive(stream, ZipArchiveMode.Create, leaveOpen: true);
 
             WriteNestJson(zipArchive);
             WritePrograms(zipArchive);