feat(io): let NestWriter keep a nest's own modified date

Write always stamped DateLastModified with the save time. That is right
for an ordinary save, but a converter carrying another system's date
cannot keep it: every PEP nest imported into the server recorded the
import time as its modified date.

PreserveDateLastModified (init, default false) writes the nest's own
date instead; every existing caller keeps stamping.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

OpenNest.IO.Tests/NestWriterModifiedDateTests.cs

@@ -0,0 +1,41 @@
+namespace OpenNest.IO.Tests;
+
+/// <summary>
+/// An ordinary save records when it happened; a converter can keep the source system's
+/// modified date through write and read.
+/// </summary>
+public class NestWriterModifiedDateTests
+{
+    private static readonly DateTime SourceDate = new(2019, 5, 2, 13, 50, 15, DateTimeKind.Unspecified);
+
+    [Fact]
+    public void Write_StampsTheSaveTimeByDefault()
+    {
+        var nest = new Nest("Stamped") { DateLastModified = SourceDate };
+        var before = DateTime.Now;
+
+        var restored = RoundTrip(new NestWriter(nest));
+
+        Assert.InRange(restored.DateLastModified, before.AddSeconds(-1), DateTime.Now.AddSeconds(1));
+        Assert.Equal(restored.DateLastModified, nest.DateLastModified);
+    }
+
+    [Fact]
+    public void Write_PreserveDateLastModified_KeepsTheNestsOwnDate()
+    {
+        var nest = new Nest("Preserved") { DateLastModified = SourceDate };
+
+        var restored = RoundTrip(new NestWriter(nest) { PreserveDateLastModified = true });
+
+        Assert.Equal(SourceDate, restored.DateLastModified);
+        Assert.Equal(SourceDate, nest.DateLastModified);
+    }
+
+    private static Nest RoundTrip(NestWriter writer)
+    {
+        using var stream = new MemoryStream();
+        Assert.True(writer.Write(stream));
+        stream.Position = 0;
+        return new NestReader(stream).Read();
+    }
+}

OpenNest.IO/NestWriter.cs

@@ -30,6 +30,13 @@ namespace OpenNest.IO
         /// </summary>
         public IBestFitStore BestFitStore { get; init; }
 
+        /// <summary>
+        /// Writes the nest's own <see cref="Nest.DateLastModified"/> instead of stamping the save
+        /// time. For converters and importers carrying another system's date; an ordinary save
+        /// leaves this false so the file records when it was saved.
+        /// </summary>
+        public bool PreserveDateLastModified { get; init; }
+
         public NestWriter(Nest nest)
         {
             this.drawingDict = new Dictionary<int, Drawing>();
@@ -44,7 +51,8 @@ namespace OpenNest.IO
 
         public bool Write(Stream stream)
         {
-            nest.DateLastModified = DateTime.Now;
+            if (!PreserveDateLastModified)
+                nest.DateLastModified = DateTime.Now;
             SetDrawingIds();
 
             using var zipArchive = new ZipArchive(stream, ZipArchiveMode.Create, leaveOpen: true);