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/cli/options.hpp

@@ -0,0 +1,30 @@
+#pragma once
+
+#include <cstdint>
+#include <optional>
+#include <string>
+#include <vector>
+
+#include "core/params.hpp"
+
+namespace effector {
+
+struct Options {
+    std::vector<std::string> images;
+    std::string effect;
+    std::string output;              // empty -> launch the interface
+    int width = 1920;
+    int height = 1080;
+    std::optional<std::uint64_t> seed;
+    ParamValues params;              // only the keys the user actually passed
+    bool listEffects = false;
+    bool showHelp = false;
+};
+
+// Throws EffectError on bad input. Effect flags are generated from the
+// registry, so a new effect gets its flags without touching this file.
+Options parseOptions(int argc, char** argv);
+
+std::string usage(const char* programName);
+
+}  // namespace effector