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/.

CMakeLists.txt

@@ -0,0 +1,52 @@
+cmake_minimum_required(VERSION 3.20)
+project(effector VERSION 0.1.0 LANGUAGES CXX)
+
+set(CMAKE_CXX_STANDARD 20)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+set(CMAKE_CXX_EXTENSIONS OFF)
+set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
+
+if(NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
+    set(CMAKE_BUILD_TYPE Release CACHE STRING "Build type" FORCE)
+endif()
+
+set(CMAKE_CXX_FLAGS_RELEASE "-O3 -DNDEBUG")
+
+find_package(PkgConfig REQUIRED)
+pkg_check_modules(GTKMM REQUIRED IMPORTED_TARGET gtkmm-4.0)
+pkg_check_modules(GDKPIXBUF REQUIRED IMPORTED_TARGET gdk-pixbuf-2.0)
+
+find_package(Threads REQUIRED)
+
+# --- core: effects and image handling, no GUI ---------------------------------
+add_library(effector_core STATIC
+    src/core/effect.cpp
+    src/core/image.cpp
+    src/core/params.cpp
+    src/core/registry.cpp
+    src/core/rng.cpp
+    src/effects/scanlines.cpp
+)
+target_include_directories(effector_core PUBLIC src)
+# The core is GUI-free: it only needs gdk-pixbuf for decoding and encoding,
+# so headless rendering works without a display.
+target_link_libraries(effector_core PUBLIC PkgConfig::GDKPIXBUF Threads::Threads)
+target_compile_options(effector_core PRIVATE -Wall -Wextra -Wpedantic)
+
+# --- application --------------------------------------------------------------
+add_executable(effector
+    src/main.cpp
+    src/cli/options.cpp
+    src/gui/app.cpp
+    src/gui/window.cpp
+)
+target_link_libraries(effector PRIVATE effector_core PkgConfig::GTKMM)
+target_compile_options(effector PRIVATE -Wall -Wextra -Wpedantic)
+
+install(TARGETS effector RUNTIME DESTINATION bin)
+
+# --- tests --------------------------------------------------------------------
+include(CTest)
+if(BUILD_TESTING)
+    add_subdirectory(tests)
+endif()