feat(render): unify physical residency accounting

src/AcDream.Content/Pak/PakReader.cs

@@ -46,6 +46,7 @@ public sealed class PakReader : IDisposable {
     private readonly ConcurrentDictionary<int, bool> _loggedCorruption = new();
 
     public PakHeader Header { get; }
+    public long FileLength => _fileLength;
 
     public PakReader(string path) {
         _fileLength = new FileInfo(path).Length;