Cascade effects through a chain

renderChain runs a list of stages in order: stage 0 works on the loaded
sources, every later one on the output of the stage before it, so blur
after scanlines softens the scanlines rather than the photographs. The
effects themselves are untouched — none of them knows whether its input
came from disk. All stages share one Rng and run in sequence, so a seed
still reproduces the cascade byte for byte.

The sidebar grows an "Effect chain" list: add a stage from a menu built
from the registry, reorder it, remove it, and edit the selected stage's
params underneath. Each stage keeps its own values. The CLI takes the
same chain from repeated -e, with every flag going to the stage whose
effect declares it.

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

src/gui/window.hpp

@@ -12,6 +12,7 @@
 #include <vector>
 
 #include "cli/options.hpp"
+#include "core/chain.hpp"
 #include "core/effect.hpp"
 #include "core/image.hpp"
 
@@ -28,6 +29,7 @@ private:
     Gtk::Widget& buildPreview();
     void buildParamWidgets();
     ParamValues paramValues() const;
+    void storeParams();     // widgets -> the selected stage's values
     void applyInitial(const Options& initial);
 
     void requestRender();   // coalesced: restarts the short debounce timer
@@ -44,7 +46,14 @@ private:
     void appendSourceRow(const std::string& path);
     void removeSource(Gtk::ListBoxRow* row);
 
-    void onEffectChanged();
+    Gtk::Widget& buildChainSection();
+    Gtk::Widget& buildAddMenu();      // the menu of effects that can be appended
+    void appendStage(const EffectPtr& effect);
+    void removeStage(int index);
+    void moveStage(int index, int delta);
+    void rebuildChainRows();
+    void selectStage(int index);
+    void onStageSelected();
     void onParamChanged();
     void onSeedEntered();
     void onRecreate();
@@ -66,7 +75,11 @@ private:
     // Model
     std::shared_ptr<const SourceList> sources_ = std::make_shared<SourceList>();
     std::vector<EffectPtr> effects_;
-    EffectPtr effect_;
+    // The cascade: stage 0 works on the sources, every later stage on the
+    // output of the one before it.
+    std::vector<ChainStage> stages_;
+    int selectedStage_ = 0;
+    bool suspendStageSelect_ = false;  // guards the rebuild of the chain rows
     Image result_;
     std::uint64_t seed_ = 0;
     bool suspendRender_ = true;
@@ -109,8 +122,10 @@ private:
     Gtk::SpinButton widthSpin_;
     Gtk::SpinButton heightSpin_;
     Gtk::Grid paramGrid_;
+    Gtk::Label paramHeading_;
     Gtk::Entry seedEntry_;
-    Gtk::DropDown effectDropdown_;
+    Gtk::ListBox chainList_;
+    Gtk::MenuButton addEffectButton_;
     Gtk::Button recreateButton_{"Recreate"};
     Gtk::Button saveButton_{"Save As…"};
     Gtk::Picture picture_;