Add project restore files and NuGet cache for GsaViewer

- Created project.nuget.cache to store NuGet package cache information.
- Added project.packagespec.json to define project restore settings and dependencies.
- Included rider.project.restore.info for Rider IDE integration.

GsaEditor.Core/IO/GsaIndexWriter.cs

@@ -0,0 +1,36 @@
+using System.Xml.Linq;
+using GsaEditor.Core.Models;
+
+namespace GsaEditor.Core.IO;
+
+/// <summary>
+/// Writes a .idx NML index file from a <see cref="GsaArchive"/>.
+/// The index file is XML-compatible and lists each entry with its alias, compression method,
+/// sizes, and data offset for fast lookup without a full sequential scan.
+/// </summary>
+public static class GsaIndexWriter
+{
+    /// <summary>
+    /// Writes the index file for the given archive to the specified path.
+    /// </summary>
+    /// <param name="archive">The archive whose entries will be indexed.</param>
+    /// <param name="filePath">The output .idx file path.</param>
+    public static void Write(GsaArchive archive, string filePath)
+    {
+        var doc = new XDocument(
+            new XElement("Index",
+                archive.Entries.Select(e =>
+                    new XElement("Entry",
+                        new XElement("Id", e.Alias),
+                        new XElement("Method", e.IsCompressed ? 1 : 0),
+                        new XElement("Len", e.OriginalLength),
+                        new XElement("CompLen", e.CompressedLength),
+                        new XElement("Offset", e.DataOffset)
+                    )
+                )
+            )
+        );
+
+        doc.Save(filePath);
+    }
+}