Fix SynchronizedSparseStream Position never advancing (regressed in 4b5b75d)

The synchronized wrapper kept its own Position auto-property and set
content.Position = Position before each operation but never wrote the advanced position
back afterward, so Position stayed frozen and reads/writes through the wrapper
re-processed the same offset (e.g. ExFat ReadLongSparseLimited read the wrong data).
Delegate Position to content.Position under the lock - correct for this single-shared-
cursor synchronized wrapper and fixes every read/write path (sync and async) at once.
Introduced by 4b5b75d ("Synchronized wrappers for disks and streams").

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: devedse

Library/DiscUtils.Streams/SparseStream.cs

@@ -656,7 +656,11 @@ private sealed class SynchronizedSparseStream(SparseStream content, Ownership ow
 
         public override long Length => content.Length;
 
-        public override long Position { get; set; }
+        public override long Position
+        {
+            get { lock (sync) { return content.Position; } }
+            set { lock (sync) { content.Position = value; } }
+        }
 
         public override void Flush()
         {