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

bin/test/data/config/world.toml

@@ -10,6 +10,7 @@ tunnel_max_distance_tiles = 10
 departure_interval_seconds = 20
 orbit_factor = 0.8
 rally_orbit_radius_tiles = 5.0
+building_blocks_tooltip = "Spend building blocks to build; deliver them to the HQ to gain more."
 
 [regions]
 asteroid_width_tiles = 40