feat(cpp/core): logger + log_window + selectable_text widgets

Logger global thread-safe con ring buffer in-memory de 2000 entradas + escritura
opcional a archivo. log_window flotante consume el ring buffer con filtros por
nivel, busqueda y autoscroll; se abre desde Settings -> Logs en la menubar.
selectable_text cubre el patron drag-to-select + Ctrl+C en cualquier ventana.

app_menubar y framework run_app integran log_window_render() en el frame loop.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cpp/framework/app_base.h

@@ -115,6 +115,13 @@ struct AppConfig {
     // llama fn_ui::app_menubar(panels, panel_count, layouts_cb) cada frame.
     fn_ui::LayoutCallbacks*   layouts_cb  = nullptr;
 
+    // Items extra dentro del menu "View", al final tras los toggles de
+    // paneles. Si view_extras != nullptr, run_app lo pasa a app_menubar.
+    // El callback se invoca dentro de un BeginMenu("View") ya abierto:
+    // la app llama directamente a ImGui::Separator(), MenuItem(), etc.
+    // NO debe abrir/cerrar el menu View.
+    std::function<bool()>     view_extras{};
+
     // Si true, run_app llama fn::gfx::gl_loader_init() tras crear el contexto
     // GL y antes del primer frame. Necesario para apps que llaman gl* directo
     // en Windows (en Linux es no-op).
@@ -136,3 +143,35 @@ int run_app(AppConfig config, std::function<void()> render_fn);
 int run_app(std::function<void()> render_fn);
 
 } // namespace fn
+
+// ----------------------------------------------------------------------------
+// E2E testing — Dear ImGui Test Engine integration.
+// ----------------------------------------------------------------------------
+//
+// Only available when the registry is built with -DFN_BUILD_TESTS=ON. The
+// CMake option defines IMGUI_ENABLE_TEST_ENGINE on imgui+fn_framework and
+// links the imgui_test_engine static lib. Without the option, run_app_test is
+// not declared and apps build identically to today.
+#ifdef IMGUI_ENABLE_TEST_ENGINE
+struct ImGuiTestEngine;
+
+namespace fn {
+
+// Run an app under Dear ImGui Test Engine. Same as run_app, but:
+//   1. Creates a test engine and binds it to the ImGui context.
+//   2. Calls register_tests(engine) once before the main loop. Tests are
+//      registered with IM_REGISTER_TEST(engine, "category", "name") and
+//      assigned a TestFunc lambda that drives the UI.
+//   3. Queues all tests matching `filter` (default "all") and ticks frames
+//      until the queue empties.
+//   4. Exits with code 0 if all tests pass, 1 if any failed or crashed.
+//
+// register_tests must be non-null and must register at least one test or the
+// function returns 1.
+int run_app_test(AppConfig config,
+                 std::function<void()> render_fn,
+                 std::function<void(ImGuiTestEngine*)> register_tests,
+                 const char* filter = "all");
+
+} // namespace fn
+#endif // IMGUI_ENABLE_TEST_ENGINE