Move chunks into the canonical source model

internal/core/source/source_test.go

@@ -222,6 +222,42 @@ func TestDigestDocumentIsDeterministicAndIncludesUnitReference(t *testing.T) {
 	}
 }
 
+func TestDigestChunkIsDeterministicAndIncludesReference(t *testing.T) {
+	doc := validDocument()
+	chunk := Chunk{
+		ID:        "chunk-1",
+		SourceID:  doc.ID,
+		Index:     0,
+		Ref:       SourceRef{SourceID: doc.ID, StartUnitID: 1, EndUnitID: 2},
+		Content:   []byte("chunk content"),
+		MediaType: "text/plain",
+		Units:     doc.Units,
+		Metadata:  map[string]any{"second": "value", "first": true},
+	}
+	first, err := DigestChunk(chunk)
+	if err != nil {
+		t.Fatalf("DigestChunk() error = %v, want nil", err)
+	}
+
+	chunk.Metadata = map[string]any{"first": true, "second": "value"}
+	second, err := DigestChunk(chunk)
+	if err != nil {
+		t.Fatalf("DigestChunk(reordered metadata) error = %v, want nil", err)
+	}
+	if first != second {
+		t.Fatalf("digests = %q and %q, want deterministic map ordering", first, second)
+	}
+
+	chunk.Ref.EndUnitID = 1
+	changed, err := DigestChunk(chunk)
+	if err != nil {
+		t.Fatalf("DigestChunk(changed ref) error = %v, want nil", err)
+	}
+	if first == changed {
+		t.Fatalf("digest = %q after reference change, want different digest", changed)
+	}
+}
+
 func TestValidateRefValid(t *testing.T) {
 	doc := validDocument()
 	ref := SourceRef{