chore(iwyu): subheaders concrets i pragma exports al umbrella

Reemplaça core/defaults.hpp pels subheaders concrets a director.cpp i
config_yaml.cpp (silencia unused-includes de clangd). Marca el umbrella
amb IWYU pragma: begin_exports/end_exports per evitar falsos positius
als consumidors transitius.

source/core/system/director.cpp

@@ -11,7 +11,8 @@
 
 #include "core/audio/audio.hpp"
 #include "core/audio/audio_adapter.hpp"
-#include "core/defaults.hpp"
+#include "core/defaults/audio.hpp"
+#include "core/defaults/window.hpp"
 #include "core/input/input.hpp"
 #include "core/input/mouse.hpp"
 #include "core/rendering/sdl_manager.hpp"