Effector: C++/CMake image effects app

Port of the Python prototype to C++20 with a CMake build:

- effector_core: GUI-free static library (effects, image IO via gdk-pixbuf,
  declarative param specs, registry, portable seeded RNG, thread helper)
- scanlines effect: plans strips from the seeded RNG, then copies pixels in
  parallel with memcpy and doubling horizontal tiling
- gtkmm 4 interface: native file dialogs, widgets generated from the param
  specs, coalesced background renders, preview as a GDK memory texture
- CLI: flags generated from the effect registry, headless rendering with no
  display required
- ctest suite covering the RNG, registry, effect geometry and image round trip

The Python prototype is kept under python-reference/.

src/gui/app.cpp

@@ -0,0 +1,19 @@
+#include "gui/app.hpp"
+
+#include <gtkmm.h>
+
+#include "gui/window.hpp"
+
+namespace effector {
+
+int runGui(const Options& initial, int argc, char** argv) {
+    // Our own parser already consumed argv, so GTK must not see it.
+    (void)argc;
+    (void)argv;
+
+    auto app = Gtk::Application::create("org.effector.Effector",
+                                        Gio::Application::Flags::NON_UNIQUE);
+    return app->make_window_and_run<EffectorWindow>(0, nullptr, initial);
+}
+
+}  // namespace effector