fix(reporting): serialize concurrent nest report exports

PDFsharp/MigraDoc layout and font state is process-wide. Parallel exports
of the same snapshot laid text out differently (for example merged words
like "Total physicalsheets" and shifted table columns). Serialize the
whole render-and-save in NestPdfWriter.Write; reports are rare.

Regression test exports one snapshot 128 times in parallel and compares
page content streams with a sequential export; it failed with dozens of
mismatches per run before the lock.

OpenNest.Tests/Reporting/NestPdfWriterConcurrencyTests.cs

@@ -0,0 +1,56 @@
+using System.Collections.Concurrent;
+using System.Text;
+using OpenNest.Reporting;
+using PdfSharp.Pdf.IO;
+
+namespace OpenNest.Tests.Reporting;
+
+public sealed class NestPdfWriterConcurrencyTests : IDisposable
+{
+    private readonly string directory = Path.Combine(Path.GetTempPath(), "opennest-pdf-concurrency-" + Guid.NewGuid().ToString("N"));
+
+    public NestPdfWriterConcurrencyTests() => Directory.CreateDirectory(directory);
+
+    public void Dispose() => Directory.Delete(directory, true);
+
+    [Fact]
+    public void Write_ConcurrentExportsProduceTheSameLayoutAsASequentialExport()
+    {
+        // PDFsharp/MigraDoc layout shares process-wide font state; unsynchronized concurrent
+        // exports produced shifted/merged words (for example "Total physicalsheets").
+        var snapshot = NestReportBuilder.Capture(NestReportTestData.CreateNest(), NestReportTestData.GeneratedAt);
+        var reference = Path.Combine(directory, "reference.pdf");
+        NestPdfWriter.Write(snapshot, reference);
+        var expected = PageContent(reference);
+        var mismatches = new ConcurrentBag<int>();
+
+        Parallel.For(0, 128, new ParallelOptions { MaxDegreeOfParallelism = System.Math.Max(8, Environment.ProcessorCount) }, index =>
+        {
+            var path = Path.Combine(directory, $"export-{index}.pdf");
+            NestPdfWriter.Write(snapshot, path);
+            if (PageContent(path) != expected)
+                mismatches.Add(index);
+        });
+
+        Assert.Empty(mismatches);
+    }
+
+    /// <summary>Page content streams only: document IDs and font-subset tags vary per export.</summary>
+    private static string PageContent(string path)
+    {
+        using var pdf = PdfReader.Open(path, PdfDocumentOpenMode.Import);
+        var text = new StringBuilder();
+        foreach (var page in pdf.Pages)
+        {
+            foreach (var item in page.Contents.Elements)
+            {
+                var dictionary = (item as PdfSharp.Pdf.Advanced.PdfReference)?.Value as PdfSharp.Pdf.PdfDictionary
+                    ?? item as PdfSharp.Pdf.PdfDictionary;
+                if (dictionary?.Stream != null)
+                    text.Append(Encoding.Latin1.GetString(dictionary.Stream.UnfilteredValue));
+            }
+            text.Append('\f');
+        }
+        return text.ToString();
+    }
+}