Unify tunnel build mode into a single Tunnel button

src/ui/BuildButtonGrid.cpp

@@ -35,11 +35,21 @@ BuildButtonGrid::BuildButtonGrid(Simulation* sim, const GameConfig* config, QWid
         {
             continue;
         }
+        // Tunnel Entry and Tunnel Exit share a single "Tunnel" button; the exit is
+        // reached through the unified tunnel build mode, not its own button
+        // (REQ-BLD-TUNNEL-MODE, REQ-UI-BUILD-GRID). Both stay player-placeable so
+        // blueprints and cost totals still account for exits.
+        if (def.type == BuildingType::TunnelExit)
+        {
+            continue;
+        }
         m_types.push_back(def.type);
         m_costs[def.type] = def.cost;
 
-        const QString label = QString::fromStdString(toDisplayName(def.id))
-            + "\n" + tr("%1 Building Blocks").arg(def.cost);
+        const QString name = (def.type == BuildingType::TunnelEntry)
+            ? tr("Tunnel")
+            : QString::fromStdString(toDisplayName(def.id));
+        const QString label = name + "\n" + tr("%1 Building Blocks").arg(def.cost);
         QPushButton* btn = new QPushButton(label, this);
         btn->setCheckable(true);
         btn->setFixedHeight(48);