extract loadUnlocks into ConfigLoaderUnlocks.cpp

Finishes the ConfigLoader.cpp domain split. ConfigLoader.cpp now holds
only the cross-domain validateUnlocks pass and loadFromDirectory
orchestrator, as intended — everything else lives in its own
ConfigLoader<Domain>.cpp. Pure move; no logic change.

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

src/lib/config/CMakeLists.txt

@@ -27,6 +27,7 @@ SET(SRCS
     ${CMAKE_CURRENT_SOURCE_DIR}/ConfigLoaderShips.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/ConfigLoaderStations.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/ConfigLoaderModules.cpp
+    ${CMAKE_CURRENT_SOURCE_DIR}/ConfigLoaderUnlocks.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/SurfaceMask.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/BlueprintSerializer.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/ShipLayoutBlueprintSerializer.cpp