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.hpp

@@ -0,0 +1,10 @@
+#pragma once
+
+#include "cli/options.hpp"
+
+namespace effector {
+
+// Runs the GTK interface with `initial` as the starting state.
+int runGui(const Options& initial, int argc, char** argv);
+
+}  // namespace effector