feat(content): add physical blob aliases to pak writer

src/AcDream.Content/Pak/PakReader.cs

@@ -170,6 +170,13 @@ public sealed class PakReader : IDisposable {
         return (long)_toc[index].Offset;
     }
 
+    /// <summary>Returns the immutable TOC receipt for alias/layout assertions in tests.</summary>
+    public PakTocEntry GetTocEntryForTest(ulong key) {
+        int index = BinarySearch(key);
+        if (index < 0) throw new KeyNotFoundException($"pak key 0x{key:X16} not found");
+        return _toc[index];
+    }
+
     /// <summary>O(n) linear scan, used only to cross-check the binary search in tests.</summary>
     public bool DebugLinearScanContainsKey(ulong key) {
         for (int i = 0; i < _toc.Length; i++) {

src/AcDream.Content/Pak/PakWriter.cs

@@ -18,6 +18,7 @@ public sealed class PakWriter : IDisposable {
     private readonly PakHeader _headerTemplate;
     private readonly List<PakTocEntry> _tocEntries = new();
     private readonly HashSet<ulong> _seenKeys = new();
+    private readonly Dictionary<ulong, PakTocEntry> _receiptByKey = new();
     private bool _finished;
 
     public PakWriter(string path, PakHeader headerTemplate) {
@@ -56,12 +57,37 @@ public sealed class PakWriter : IDisposable {
         _stream.Write(bytes);
         PadToAlignment();
 
-        _tocEntries.Add(new PakTocEntry {
+        var receipt = new PakTocEntry {
             Key = key,
             Offset = (ulong)offset,
             Length = (uint)bytes.Length,
             Crc32 = Content.Pak.Crc32.Compute(bytes),
-        });
+        };
+        _tocEntries.Add(receipt);
+        _receiptByKey.Add(key, receipt);
+    }
+
+    /// <summary>
+    /// Adds a second independently addressable key for an existing physical
+    /// blob. The alias receives a normal TOC row with the source blob's exact
+    /// offset, length, and CRC; no payload bytes are written again.
+    /// </summary>
+    public void AddAlias(ulong aliasKey, ulong existingKey) {
+        ThrowIfFinished();
+        if (aliasKey == existingKey) {
+            throw new ArgumentException("an alias key must differ from its source key", nameof(aliasKey));
+        }
+        if (!_receiptByKey.TryGetValue(existingKey, out var source)) {
+            throw new KeyNotFoundException($"source pak key 0x{existingKey:X16} has not been written");
+        }
+        if (!_seenKeys.Add(aliasKey)) {
+            throw new ArgumentException($"duplicate pak key 0x{aliasKey:X16}", nameof(aliasKey));
+        }
+
+        var alias = source;
+        alias.Key = aliasKey;
+        _tocEntries.Add(alias);
+        _receiptByKey.Add(aliasKey, alias);
     }
 
     /// <summary>Writes the sorted TOC and finalizes the header. Must be called exactly once.</summary>