Let several text stages share one named look

A picture with a title, a caption and a credit meant typing the same
font, size and colour into every text stage, and changing the look meant
typing them all again. A project now holds named styles: a stage follows
one, and editing it moves every stage that follows it at once.

The front-ends learn nothing effect-specific. A ParamSpec can be marked
.styled(), saying a style carries it rather than the stage, and
Effect::styleKind() names the pool it comes from -- so styleSpecs(kind)
is to a style what params() is to a stage, and a second text-ish effect
would slot into the styles that already exist. The look is what a style
carries; the words and the placement stay on the stage, which is what
lets two stages share a typography in two different corners.

stageValues() resolves one in a single place, and a stage keeps a full
set of values of its own beside the style, so switching it to Custom,
dropping the style, or opening the project in a build that does not know
it renders exactly the same picture. A style moves stages together; it
never holds the only copy of anything.

The command line builds no styles and passes none, so it renders what it
always did.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/gui/window.hpp

@@ -7,6 +7,7 @@
 #include <memory>
 #include <mutex>
 #include <optional>
+#include <functional>
 #include <string>
 #include <thread>
 #include <vector>
@@ -16,6 +17,7 @@
 #include "core/effect.hpp"
 #include "core/image.hpp"
 #include "core/project.hpp"
+#include "core/style.hpp"
 
 namespace effector {
 
@@ -30,7 +32,29 @@ private:
     Gtk::Widget& buildPreview();
     void buildParamWidgets();
     ParamValues paramValues() const;
-    void storeParams();     // widgets -> the selected stage's values
+    void storeParams();     // widgets -> the selected stage, and its style
+
+    // The style row above the params of a stage whose effect takes one.
+    std::unique_ptr<Gtk::Widget> buildStyleRow(const ChainStage& stage);
+    // The names of the pool the selected stage draws from, in the order the
+    // dropdown shows them.
+    std::vector<std::string> styleNames(const std::string& kind) const;
+    // The style the selected stage follows, or nullptr when it stands alone.
+    Style* selectedStyle();
+    const Style* selectedStyle() const;
+    void onStyleSelected();   // the dropdown: adopt, drop, or make a new one
+    void adoptStyle(const std::string& name);
+    void onNewStyle();
+    void onRenameStyle();
+    void onRemoveStyle();
+    // The little modal that asks for a style name, used by both New and
+    // Rename. Built from scratch each time, like the progress window.
+    void askStyleName(const std::string& title, const std::string& initial,
+                      const std::function<void(const std::string&)>& accept);
+    void closeStyleWindow();
+    // A name that is empty, or already used in the pool, is refused in place.
+    bool styleNameTaken(const std::string& kind, const std::string& name) const;
+
     void applyInitial(const Options& initial);
 
     bool canRender() const;     // a chain, and sources if the chain needs them
@@ -114,6 +138,9 @@ private:
     // The cascade: stage 0 works on the sources, every later stage on the
     // output of the one before it.
     std::vector<ChainStage> stages_;
+    // The named looks the stages share. Edited through the params of whichever
+    // stage follows one, and saved with the project.
+    StyleSet styles_;
     int selectedStage_ = 0;
     bool suspendStageSelect_ = false;  // guards the rebuild of the chain rows
     Image result_;
@@ -166,6 +193,11 @@ private:
     // Set while a format fills the spins, so their own handler does not queue
     // a second render on top of the one the format change asks for.
     bool suspendCanvasSync_ = false;
+    Gtk::DropDown* styleDrop_ = nullptr;   // null while the stage takes no style
+    bool suspendStyleSelect_ = false;      // guards filling the dropdown
+    std::shared_ptr<Gtk::Window> styleWindow_;
+    Gtk::Entry* styleNameEntry_ = nullptr;  // nulled with the window
+    Gtk::Label* styleNameError_ = nullptr;
     Gtk::Grid paramGrid_;
     Gtk::Label paramHeading_;
     Gtk::Entry seedEntry_;