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/GsaFormat.cs

@@ -0,0 +1,18 @@
+namespace GsaEditor.Core.Models;
+
+/// <summary>
+/// Identifies the archive format variant.
+/// </summary>
+public enum GsaFormat
+{
+    /// <summary>
+    /// Legacy format with no padding support. Magic word = 0x4E415243 ("NARC").
+    /// </summary>
+    NARC,
+
+    /// <summary>
+    /// Enhanced format with offset and size padding for console DVD alignment.
+    /// Magic word = 0x4E415244 ("NARD").
+    /// </summary>
+    NARD
+}