extract shared TOML helpers into TomlHelpers.h/.cpp

ConfigLoader.cpp had grown to 877 lines by mixing generic TOML-parsing
helpers (used by every per-file loader) with per-domain parsing logic.
Splitting the file per config domain first requires pulling out the
helpers shared by two or more domains, so each domain .cpp can include
them without duplication. Pure move: no logic, message, or ordering
changes.

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

src/lib/config/CMakeLists.txt

@@ -13,6 +13,7 @@ SET(HDRS
     ${CMAKE_CURRENT_SOURCE_DIR}/SurfaceMask.h
     ${CMAKE_CURRENT_SOURCE_DIR}/BlueprintSerializer.h
     ${CMAKE_CURRENT_SOURCE_DIR}/ShipLayoutBlueprintSerializer.h
+    ${CMAKE_CURRENT_SOURCE_DIR}/TomlHelpers.h
     PARENT_SCOPE
 )
 
@@ -23,6 +24,7 @@ SET(SRCS
     ${CMAKE_CURRENT_SOURCE_DIR}/SurfaceMask.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/BlueprintSerializer.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/ShipLayoutBlueprintSerializer.cpp
+    ${CMAKE_CURRENT_SOURCE_DIR}/TomlHelpers.cpp
     PARENT_SCOPE
 )