Implement REQ-UI-BLOCKS-TOOLTIP: an optional world.toml [world].building_blocks_tooltip, threaded into HeaderBar via a new const GameConfig* parameter and shown as the building blocks stock label's hover tooltip. Omitted when unset. Author the text in the app config and extend ConfigLoaderTest to cover the field. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01DZR44tA8sn4dPqDzAVXyps