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();
+}

GsaEditor.Core/Models/GsaEntry.cs

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+using GsaEditor.Core.Compression;
+
+namespace GsaEditor.Core.Models;
+
+/// <summary>
+/// Represents a single file entry within a GSA archive.
+/// </summary>
+public class GsaEntry
+{
+    /// <summary>
+    /// Relative path (alias) of the file within the archive, using '/' as separator.
+    /// Maximum 511 characters.
+    /// </summary>
+    public string Alias { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Whether this entry's data is zlib-compressed.
+    /// </summary>
+    public bool IsCompressed { get; set; }
+
+    /// <summary>
+    /// Original (decompressed) size of the file in bytes.
+    /// </summary>
+    public uint OriginalLength { get; set; }
+
+    /// <summary>
+    /// Compressed size of the file in bytes. Equal to <see cref="OriginalLength"/> if not compressed.
+    /// </summary>
+    public uint CompressedLength { get; set; }
+
+    /// <summary>
+    /// Raw data as stored in the archive (compressed bytes if <see cref="IsCompressed"/> is true).
+    /// </summary>
+    public byte[] RawData { get; set; } = Array.Empty<byte>();
+
+    /// <summary>
+    /// Byte offset of this entry's data block within the archive file.
+    /// </summary>
+    public long DataOffset { get; set; }
+
+    /// <summary>
+    /// Returns the decompressed data for this entry.
+    /// If the entry is not compressed, returns <see cref="RawData"/> directly.
+    /// </summary>
+    /// <returns>The decompressed file content.</returns>
+    public byte[] GetDecompressedData()
+    {
+        if (!IsCompressed)
+            return RawData;
+
+        return ZlibHelper.Decompress(RawData, OriginalLength);
+    }
+
+    /// <summary>
+    /// Sets the entry data from uncompressed bytes, optionally compressing with zlib.
+    /// Updates <see cref="OriginalLength"/>, <see cref="CompressedLength"/>,
+    /// <see cref="IsCompressed"/>, and <see cref="RawData"/>.
+    /// </summary>
+    /// <param name="uncompressedData">The uncompressed file content.</param>
+    /// <param name="compress">Whether to compress the data with zlib.</param>
+    public void SetData(byte[] uncompressedData, bool compress)
+    {
+        OriginalLength = (uint)uncompressedData.Length;
+        IsCompressed = compress;
+
+        if (compress)
+        {
+            RawData = ZlibHelper.Compress(uncompressedData);
+            CompressedLength = (uint)RawData.Length;
+        }
+        else
+        {
+            RawData = uncompressedData;
+            CompressedLength = OriginalLength;
+        }
+    }
+}

GsaEditor.Core/Models/GsaFormat.cs

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+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
+}