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/params.hpp

@@ -0,0 +1,49 @@
+#pragma once
+
+#include <map>
+#include <string>
+#include <variant>
+#include <vector>
+
+namespace effector {
+
+// Declarative description of one effect knob. The GUI turns these into widgets
+// and the CLI turns them into flags, so neither knows about any effect.
+struct ParamSpec {
+    enum class Type { Int, Double, Bool, Choice };
+
+    Type type = Type::Int;
+    std::string key;
+    std::string label;
+    std::string help;
+    double defaultValue = 0.0;
+    double minimum = 0.0;
+    double maximum = 0.0;
+    double step = 1.0;
+    int digits = 2;
+    std::string defaultChoice;
+    std::vector<std::string> choices;
+
+    std::string flag() const;  // "strip_height" -> "--strip-height"
+
+    static ParamSpec makeInt(std::string key, std::string label, long defaultValue, long minimum,
+                             long maximum, std::string help = {}, long step = 1);
+    static ParamSpec makeDouble(std::string key, std::string label, double defaultValue,
+                                double minimum, double maximum, std::string help = {},
+                                double step = 0.05, int digits = 2);
+    static ParamSpec makeBool(std::string key, std::string label, bool defaultValue,
+                              std::string help = {});
+    static ParamSpec makeChoice(std::string key, std::string label, std::string defaultChoice,
+                                std::vector<std::string> choices, std::string help = {});
+};
+
+using ParamValue = std::variant<long, double, bool, std::string>;
+using ParamValues = std::map<std::string, ParamValue>;
+
+long paramInt(const ParamValues& values, const std::string& key, long fallback);
+double paramDouble(const ParamValues& values, const std::string& key, double fallback);
+bool paramBool(const ParamValues& values, const std::string& key, bool fallback);
+std::string paramChoice(const ParamValues& values, const std::string& key,
+                        const std::string& fallback);
+
+}  // namespace effector