Label text stages in the chain list with their words

Effect::label(values) names a stage in a list, defaulting to the title;
the text effect answers "Text (words)", and the chain rows are renamed
in place as params change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/core_tests.cpp

@@ -857,6 +857,13 @@ int main() {
         // Like blur and overlay it needs no sources of its own, and the same
         // settings render the same bytes twice over.
         check(!TextEffect().needsSources(), "text can lead a chain");
+
+        // A text stage is named after its words, on one line.
+        check(TextEffect().label({{"text", std::string("  Two\nlines ")}}) == "Text (Two lines)",
+              "a text stage is labelled with its words");
+        check(TextEffect().label({{"text", std::string(" \n")}}) == "Text",
+              "a text stage with no words is labelled with the title alone");
+        check(effectByName("blur")->label({}) == "Blur", "other effects are labelled with their title");
         const auto alone = renderText({}, 200, 120, plain);
         check(alone.width() == 200 && countRed(alone, 0, 0, 200, 120) > 0,
               "without sources it draws on an empty canvas");