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/Models/GsaArchive.cs

@@ -0,0 +1,27 @@
+namespace GsaEditor.Core.Models;
+
+/// <summary>
+/// Represents a loaded GSA archive with its global header information and file entries.
+/// </summary>
+public class GsaArchive
+{
+    /// <summary>
+    /// The format variant of this archive (NARC or NARD).
+    /// </summary>
+    public GsaFormat Format { get; set; } = GsaFormat.NARC;
+
+    /// <summary>
+    /// (NARD only) Byte alignment for data offsets. Zero or unused for NARC.
+    /// </summary>
+    public int OffsetPadding { get; set; }
+
+    /// <summary>
+    /// (NARD only) Byte alignment for data sizes. Zero or unused for NARC.
+    /// </summary>
+    public int SizePadding { get; set; }
+
+    /// <summary>
+    /// The ordered list of file entries in the archive.
+    /// </summary>
+    public List<GsaEntry> Entries { get; set; } = new();
+}