Dispose the streams of a ReadOnlyCompositeStream (#6)

Disposing a ReadOnlyCompositeStream did not dispose the streams it
reads from, so files opened only for the composite stayed open until
their streams were finalized. WiseOverlayHeader.OpenFile in
SabreTools.Serialization opens a Wise installer and all of its parts
this way.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

SabreTools.IO.Test/ReadOnlyCompositeStreamTests.cs

@@ -158,6 +158,24 @@ namespace SabreTools.IO.Test
 
         #endregion
 
+        #region Dispose
+
+        [Fact]
+        public void Dispose_DisposesStreams()
+        {
+            var first = new MemoryStream(new byte[128]);
+            var second = new MemoryStream(new byte[128]);
+            var stream = new ReadOnlyCompositeStream(first);
+            stream.AddStream(second);
+
+            stream.Dispose();
+
+            Assert.False(first.CanRead);
+            Assert.False(second.CanRead);
+        }
+
+        #endregion
+
         #region Unimplemented
 
         [Fact]

SabreTools.IO/ReadOnlyCompositeStream.cs

@@ -163,6 +163,20 @@ namespace SabreTools.IO
 
         #region Stream Implementations
 
+        /// <inheritdoc/>
+        protected override void Dispose(bool disposing)
+        {
+            if (disposing)
+            {
+                foreach (var stream in _streams)
+                {
+                    stream.Dispose();
+                }
+            }
+
+            base.Dispose(disposing);
+        }
+
         /// <inheritdoc/>
         public override void Flush()
             => throw new NotImplementedException();