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.cpp

@@ -0,0 +1,26 @@
+#include "core/effect.hpp"
+
+namespace effector {
+
+ParamValues Effect::defaults() const {
+    ParamValues values;
+    for (const auto& spec : params()) {
+        switch (spec.type) {
+            case ParamSpec::Type::Int:
+                values[spec.key] = static_cast<long>(spec.defaultValue);
+                break;
+            case ParamSpec::Type::Double:
+                values[spec.key] = spec.defaultValue;
+                break;
+            case ParamSpec::Type::Bool:
+                values[spec.key] = spec.defaultValue != 0.0;
+                break;
+            case ParamSpec::Type::Choice:
+                values[spec.key] = spec.defaultChoice;
+                break;
+        }
+    }
+    return values;
+}
+
+}  // namespace effector