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/main.cpp

@@ -0,0 +1,70 @@
+#include <chrono>
+#include <cstdio>
+#include <iomanip>
+#include <iostream>
+
+#include "cli/options.hpp"
+#include "core/effect.hpp"
+#include "core/image.hpp"
+#include "core/registry.hpp"
+#include "core/rng.hpp"
+#include "gui/app.hpp"
+
+namespace {
+
+int renderHeadless(const effector::Options& options) {
+    const auto effect = effector::effectByName(options.effect);
+
+    std::vector<effector::LoadFailure> failures;
+    auto sources = effector::loadImages(options.images, failures);
+    for (const auto& failure : failures) {
+        std::cerr << "effector: skipping " << failure.path << ": " << failure.message << "\n";
+    }
+    if (sources.empty()) {
+        std::cerr << "effector: no usable source images\n";
+        return 2;
+    }
+
+    const std::uint64_t seed = options.seed.value_or(effector::Rng::randomSeed());
+    effector::Rng rng(seed);
+
+    auto values = effect->defaults();
+    for (const auto& [key, value] : options.params) values[key] = value;
+
+    const auto start = std::chrono::steady_clock::now();
+    const auto image = effect->render(sources, options.width, options.height, rng, values);
+    const auto renderMillis =
+        std::chrono::duration<double, std::milli>(std::chrono::steady_clock::now() - start).count();
+    image.save(options.output);
+
+    std::cout << options.output << "  " << options.width << "x" << options.height
+              << "  effect=" << effect->name() << "  seed=" << seed << "  render="
+              << std::fixed << std::setprecision(1) << renderMillis << " ms\n";
+    return 0;
+}
+
+}  // namespace
+
+int main(int argc, char** argv) {
+    try {
+        const auto options = effector::parseOptions(argc, argv);
+
+        if (options.showHelp) {
+            std::cout << effector::usage(argv[0]);
+            return 0;
+        }
+        if (options.listEffects) {
+            for (const auto& effect : effector::effects()) {
+                std::cout << effect->name() << "\t" << effect->title() << " - "
+                          << effect->description() << "\n";
+            }
+            return 0;
+        }
+        if (!options.output.empty()) return renderHeadless(options);
+
+        return effector::runGui(options, argc, argv);
+    } catch (const std::exception& error) {
+        std::cerr << "effector: " << error.what() << "\n";
+        return 1;
+    }
+}