Move chunks into the canonical source model

internal/modules/dnd/chunk/scenes/chunker_test.go

@@ -179,8 +179,8 @@ func TestChunkReturnsSceneChunksFromStructuredOutput(t *testing.T) {
 	if first.SourceID != "session-alpha" || first.Index != 0 {
 		t.Fatalf("first chunk = %#v, want source and index fields", first)
 	}
-	if first.StartUnitID != 1 || first.EndUnitID != 2 {
-		t.Fatalf("first boundaries = %d-%d, want 1-2", first.StartUnitID, first.EndUnitID)
+	if first.Ref != (source.SourceRef{SourceID: "session-alpha", StartUnitID: 1, EndUnitID: 2}) {
+		t.Fatalf("first ref = %#v, want session-alpha:1-2", first.Ref)
 	}
 	if first.MediaType != "application/json" || len(first.Content) == 0 {
 		t.Fatalf("first payload = media type %q length %d, want JSON content", first.MediaType, len(first.Content))
@@ -574,7 +574,7 @@ func scene(startUnitID int, endUnitID int) sceneResponse {
 	}
 }
 
-func chunkIDs(chunks []contracts.SourceChunk) []string {
+func chunkIDs(chunks []source.Chunk) []string {
 	ids := make([]string, 0, len(chunks))
 	for _, chunk := range chunks {
 		ids = append(ids, chunk.ID)