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/effect.hpp

@@ -0,0 +1,38 @@
+#pragma once
+
+#include <memory>
+#include <stdexcept>
+#include <string>
+#include <vector>
+
+#include "core/image.hpp"
+#include "core/params.hpp"
+#include "core/rng.hpp"
+
+namespace effector {
+
+class EffectError : public std::runtime_error {
+public:
+    using std::runtime_error::runtime_error;
+};
+
+// Base class for every effect. Canvas size and the random source are common to
+// all of them; params() covers only the effect-specific knobs.
+class Effect {
+public:
+    virtual ~Effect() = default;
+
+    virtual std::string name() const = 0;
+    virtual std::string title() const = 0;
+    virtual std::string description() const = 0;
+    virtual const std::vector<ParamSpec>& params() const = 0;
+
+    virtual Image render(const std::vector<Image>& sources, int width, int height, Rng& rng,
+                         const ParamValues& values) const = 0;
+
+    ParamValues defaults() const;
+};
+
+using EffectPtr = std::shared_ptr<const Effect>;
+
+}  // namespace effector