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/game/config_yaml.cpp

@@ -5,7 +5,8 @@
 #include <string>
 #include <unordered_map>
 
-#include "core/defaults.hpp"
+#include "core/defaults/rendering.hpp"
+#include "core/defaults/window.hpp"
 #include "external/fkyaml_node.hpp"
 #include "project.h"