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/core/registry.cpp

@@ -0,0 +1,29 @@
+#include "core/registry.hpp"
+
+#include "effects/scanlines.hpp"
+
+namespace effector {
+
+// To add an effect: include its header and add one line here.
+const std::vector<EffectPtr>& effects() {
+    static const std::vector<EffectPtr> registry = {
+        std::make_shared<ScanlinesEffect>(),
+    };
+    return registry;
+}
+
+EffectPtr effectByName(const std::string& name) {
+    for (const auto& effect : effects()) {
+        if (effect->name() == name) return effect;
+    }
+    std::string known;
+    for (const auto& effect : effects()) {
+        if (!known.empty()) known += ", ";
+        known += effect->name();
+    }
+    throw EffectError("Unknown effect '" + name + "' (available: " + known + ")");
+}
+
+std::string defaultEffectName() { return effects().front()->name(); }
+
+}  // namespace effector