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/GsaIndexReader.cs

@@ -0,0 +1,70 @@
+using System.Xml.Linq;
+
+namespace GsaEditor.Core.IO;
+
+/// <summary>
+/// Represents a single entry parsed from a .idx NML index file.
+/// </summary>
+public class GsaIndexEntry
+{
+    /// <summary>
+    /// The file alias (relative path) of the entry.
+    /// </summary>
+    public string Id { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Compression method: 0 = raw, 1 = zlib.
+    /// </summary>
+    public int Method { get; set; }
+
+    /// <summary>
+    /// Original (decompressed) size in bytes.
+    /// </summary>
+    public uint Length { get; set; }
+
+    /// <summary>
+    /// Compressed size in bytes.
+    /// </summary>
+    public uint CompressedLength { get; set; }
+
+    /// <summary>
+    /// Byte offset of the data block within the archive.
+    /// </summary>
+    public long Offset { get; set; }
+}
+
+/// <summary>
+/// Parses .idx NML index files (XML-compatible format) into a list of <see cref="GsaIndexEntry"/>.
+/// </summary>
+public static class GsaIndexReader
+{
+    /// <summary>
+    /// Reads index entries from the specified .idx file.
+    /// </summary>
+    /// <param name="filePath">The path to the .idx file.</param>
+    /// <returns>A list of parsed index entries.</returns>
+    public static List<GsaIndexEntry> Read(string filePath)
+    {
+        var doc = XDocument.Load(filePath);
+        var entries = new List<GsaIndexEntry>();
+
+        var root = doc.Root;
+        if (root == null || root.Name.LocalName != "Index")
+            return entries;
+
+        foreach (var entryEl in root.Elements("Entry"))
+        {
+            var entry = new GsaIndexEntry
+            {
+                Id = entryEl.Element("Id")?.Value ?? string.Empty,
+                Method = int.TryParse(entryEl.Element("Method")?.Value, out var m) ? m : 0,
+                Length = uint.TryParse(entryEl.Element("Len")?.Value, out var l) ? l : 0,
+                CompressedLength = uint.TryParse(entryEl.Element("CompLen")?.Value, out var cl) ? cl : 0,
+                Offset = long.TryParse(entryEl.Element("Offset")?.Value, out var o) ? o : 0
+            };
+            entries.Add(entry);
+        }
+
+        return entries;
+    }
+}