Move chunks into the canonical source model

internal/cli/compatibility_test.go

@@ -330,8 +330,8 @@ func TestProductionLLMCallersShareScheduledClient(t *testing.T) {
 			{ID: 2, Kind: "segment", Text: "The spell takes effect.", Ref: source.SourceRef{SourceID: "session-alpha", StartUnitID: 2, EndUnitID: 2}},
 		},
 	}
-	chunk := contracts.SourceChunk{
-		ID: "session-alpha:chunk:0", SourceID: doc.ID, Index: 0, StartUnitID: 1, EndUnitID: 2,
+	chunk := source.Chunk{
+		ID: "session-alpha:chunk:0", SourceID: doc.ID, Index: 0, Ref: source.SourceRef{SourceID: doc.ID, StartUnitID: 1, EndUnitID: 2},
 		Content: []byte(`{"scene":"Aria casts Cure Wounds."}`), MediaType: "application/json", Units: append([]source.SourceUnit(nil), doc.Units...),
 	}