Avoid redundant decoded graph clones

internal/framework/evidencecontext/evidencecontext_test.go

@@ -180,12 +180,12 @@ func TestCodecRoundTripsCompactFixtureAndOwnsDecodedValues(t *testing.T) {
 		t.Fatalf("fixture does not use canonical encoding\nwant: %s\n got: %s", fixture, encoded)
 	}
 	value.Contexts[0].Units[0].Text = "changed"
-	decoded, err := codec.Decode(fixture)
+	decoded, err := codec.Decode(encoded)
 	if err != nil {
 		t.Fatal(err)
 	}
 	if decoded.Contexts[0].Units[0].Text != "The party meets Rowan." {
-		t.Fatal("Decode() reused mutable document storage")
+		t.Fatal("Encode() retained mutable document storage")
 	}
 	built, err := Build(BuildRequest{Source: testDocument(t), WindowUnits: 1, SelectedLanes: []string{"npcs"}, LaneEvidence: []LaneEvidence{{LaneID: "npcs", SourceRefs: []source.SourceRef{ref(3, 3)}}}})
 	if err != nil {