fix: make ZstdSkippableContent defensively copy its bytes (#21)

The record stored the caller's array and handed it back from content()
uncopied, so a "value" object could be mutated through the input array
or the accessor result. Clone in the compact constructor and in the
content() accessor, matching ZstdDictionary's copy-in/copy-out. Add a
test that mutating the source array and the accessor result leaves the
record untouched.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: dfa1

zstd/src/main/java/io/github/dfa1/zstd/ZstdSkippableContent.java

@@ -9,6 +9,20 @@
 /// @param magicVariant the variant 0..15 the frame was written with
 public record ZstdSkippableContent(byte[] content, int magicVariant) {
 
+    /// Defensively copies `content` so the record owns its bytes and cannot be
+    /// mutated through the array the caller passed in.
+    public ZstdSkippableContent {
+        content = content.clone();
+    }
+
+    /// The embedded content bytes, as a fresh copy so the record stays immutable.
+    ///
+    /// @return a copy of the content bytes
+    @Override
+    public byte[] content() {
+        return content.clone();
+    }
+
     /// Value equality over the payload and variant, comparing `content` by its
     /// bytes rather than by array identity (the record default).
     ///

zstd/src/test/java/io/github/dfa1/zstd/ZstdFrameTest.java

@@ -142,6 +142,20 @@ void standardFrameIsNotSkippable() {
             assertThat(ZstdFrame.isSkippableFrame(Zstd.compress(PAYLOAD))).isFalse();
         }
 
+        @Test
+        void defensivelyCopiesContentInAndOut() {
+            // Given a backing array wrapped in a skippable-content value
+            byte[] backing = "metadata".getBytes(StandardCharsets.UTF_8);
+            ZstdSkippableContent content = new ZstdSkippableContent(backing, 2);
+
+            // When the source array and a value returned by the accessor are mutated
+            backing[0] = 'X';
+            content.content()[1] = 'X';
+
+            // Then the record's own bytes are untouched
+            assertThat(content.content()).isEqualTo("metadata".getBytes(StandardCharsets.UTF_8));
+        }
+
         @Test
         void contentHasValueEqualityOverTheBytesNotArrayIdentity() {
             // Given two separately built payloads with the same bytes and variant, and one differing