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>

src/core/effect.hpp

@@ -38,6 +38,13 @@ public:
     // use. An effect with no kind must declare no styled params.
     virtual std::string styleKind() const { return {}; }
 
+    // How a stage of this effect is named in a list of stages, given its
+    // values: the title unless the values say something more telling.
+    virtual std::string label(const ParamValues& values) const {
+        (void)values;
+        return title();
+    }
+
     virtual Image render(const std::vector<Image>& sources, int width, int height, Rng& rng,
                          const ParamValues& values) const = 0;