extract loadBuildings into ConfigLoaderBuildings.cpp

Continues the ConfigLoader.cpp domain split. Pure move of loadBuildings;
no domain-specific helpers to relocate here.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YHcUerKAZKWNvSKJxYKbnG

src/lib/config/CMakeLists.txt

@@ -22,6 +22,7 @@ SET(SRCS
     ${CMAKE_CURRENT_SOURCE_DIR}/Formula.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/ConfigLoader.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/ConfigLoaderWorld.cpp
+    ${CMAKE_CURRENT_SOURCE_DIR}/ConfigLoaderBuildings.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/SurfaceMask.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/BlueprintSerializer.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/ShipLayoutBlueprintSerializer.cpp