Avoid redundant decoded graph clones

internal/framework/chunkmap/codec.go

@@ -102,11 +102,11 @@ func Build(request BuildRequest) (ChunkMap, error) {
 			Annotations: source.CloneChunkAnnotations(chunk.Annotations),
 		}
 	}
-	canonical, err := canonicalize(value)
+	canonical, err := canonicalizeOwned(value)
 	if err != nil {
 		return ChunkMap{}, fmt.Errorf("validate chunk map: %w", err)
 	}
-	return clone(canonical), nil
+	return canonical, nil
 }
 
 // Serialize builds and encodes the framework-owned serialized artifact.
@@ -132,7 +132,7 @@ func (c *Codec) Encode(value ChunkMap) ([]byte, error) {
 	if _, err := c.schemaBytes(); err != nil {
 		return nil, err
 	}
-	canonical, err := canonicalize(clone(value))
+	canonical, err := canonicalize(value)
 	if err != nil {
 		return nil, fmt.Errorf("encode source chunk map: %w", err)
 	}
@@ -163,11 +163,11 @@ func (c *Codec) Decode(content []byte) (ChunkMap, error) {
 	if err := decoder.Decode(&trailing); err != io.EOF {
 		return ChunkMap{}, fmt.Errorf("decode source chunk map: multiple JSON values")
 	}
-	canonical, err := canonicalize(value)
+	canonical, err := canonicalizeOwned(value)
 	if err != nil {
 		return ChunkMap{}, fmt.Errorf("decode source chunk map: %w", err)
 	}
-	return clone(canonical), nil
+	return canonical, nil
 }
 
 func (c *Codec) schemaBytes() ([]byte, error) {
@@ -241,6 +241,10 @@ func hasRequiredFields(required []string) bool {
 }
 
 func canonicalize(value ChunkMap) (ChunkMap, error) {
+	return canonicalizeOwned(clone(value))
+}
+
+func canonicalizeOwned(value ChunkMap) (ChunkMap, error) {
 	if err := requireIdentity("source_id", value.SourceID); err != nil {
 		return ChunkMap{}, err
 	}

internal/framework/chunkmap/codec_test.go

@@ -56,6 +56,15 @@ func TestCodecRoundTripsValidFixture(t *testing.T) {
 	if !bytes.Equal(encoded, bytes.TrimSpace(fixture)) {
 		t.Fatalf("fixture does not use canonical encoding\nwant: %s\n got: %s", fixture, encoded)
 	}
+	value.PlanAnnotations["test/chunker"][0] = '['
+	value.Chunks[0].Annotations["test/chunker"][0] = '['
+	decoded, err := codec.Decode(encoded)
+	if err != nil {
+		t.Fatalf("Decode(encoded) after mutation error = %v", err)
+	}
+	if string(decoded.PlanAnnotations["test/chunker"]) != `{"label":"fixture"}` || string(decoded.Chunks[0].Annotations["test/chunker"]) != `{"category":"sample"}` {
+		t.Fatalf("Decode() reused mutable chunk-map storage: %#v", decoded)
+	}
 }
 
 func TestBuildCanonicalizesAnnotationFormatting(t *testing.T) {