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/test/ConfigLoaderTest.cpp

@@ -83,6 +83,11 @@ TEST_CASE("ConfigLoader loads the committed bin/config/ configs end-to-end", "[c
     REQUIRE(cfg.world.rallyOrbitRadius_tiles == Approx(5.0));
     REQUIRE(cfg.world.scrapPerThreat == Approx(1.0));
 
+    // Optional header building blocks tooltip (REQ-UI-BLOCKS-TOOLTIP).
+    REQUIRE(cfg.world.buildingBlocksTooltip.has_value());
+    REQUIRE(*cfg.world.buildingBlocksTooltip ==
+            "Spend building blocks to build; deliver them to the HQ to gain more.");
+
     // Spot-check that a config-derived formula computes as expected.
     // threat_rate_formula = "x": evaluates to the input value.
     REQUIRE(cfg.world.waves.threatRateFormula.evaluate(1.0) == Approx(1.0));