Add config-defined tooltip to header building blocks stock

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

src/ui/HeaderBar.h

@@ -9,6 +9,7 @@
 #include "BuildingBlocksChangedEvent.h"
 #include "EventHandler.h"
 #include "ExpansionCostChangedEvent.h"
+#include "GameConfig.h"
 #include "GameSpeedChangedEvent.h"
 #include "Tick.h"
 #include "TickAdvancedEvent.h"
@@ -27,7 +28,7 @@ class HeaderBar : public QWidget,
     Q_OBJECT
 
 public:
-    explicit HeaderBar(QWidget* parent = nullptr);
+    explicit HeaderBar(const GameConfig* config, QWidget* parent = nullptr);
     ~HeaderBar() override;
 
 private slots: