Build menu: traps, doors, steel walls and siege engines; new tools unlock by mission

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/research.rs

@@ -207,6 +207,14 @@ mod tests {
         assert!(!g.can_research(31), "magic doors are far away");
     }
 
+    #[test]
+    fn siege_engines_are_not_known_in_the_first_missions() {
+        let g = crate::campaign::start_mission(0, None);
+        for t in [19, 47, 48, 50, 51] {
+            assert!(!g.tool_allowed(t), "tool {} known in mission 1", t);
+        }
+    }
+
     #[test]
     fn once_everything_is_known_the_library_raises_room_levels() {
         let mut g = Game::new(9, 64, 64);