explicit recipe unlocking

src/lib/config/RecipesConfig.h

@@ -32,6 +32,10 @@ struct RecipeDef
     std::vector<RecipeIngredient> inputs;
     std::vector<RecipeOutput> outputs;
     double durationSeconds;
+    // Assembler only. nullopt = implicit-only locking. -1 = explicitly unlocked
+    // at game start. >= 0 = locked; schematic enters drop pool at that station
+    // level once the output item is implicitly unlocked (REQ-LOCK-EXPLICIT).
+    std::optional<int> unlockAtStationLevel;
 };
 
 struct RecipesConfig