Tornado disasters + settings toggle

- Monthly-chance tornadoes (after month 30 grace) carve a wide erratic
  path, flattening buildings into rubble; anchors stay consistent
- 'Tornado disasters' toggle in menu settings, persisted
- Help handbook documents the hazard and the rebuild loop
- Engine suite grows to 45 assertions (spawn damage, event, anchors,
  suppression when disabled); soak keeps economy focus disaster-free

src/main.js

@@ -49,6 +49,7 @@ class Game {
   // ---------- lifecycle ----------
 
   start(city) {
+    city.disastersEnabled = this.settings.disasters !== false;
     this.teardown();
     this.city = city;
 
@@ -131,6 +132,7 @@ class Game {
     Object.assign(this.settings, patch);
     localStorage.setItem(SETTINGS_KEY, JSON.stringify(this.settings));
     if ('shadows' in patch && this._parts) this._parts.renderer.setShadows(patch.shadows);
+    if ('disasters' in patch && this.city) this.city.disastersEnabled = patch.disasters !== false;
   }
 
   queryTile(tile, x, y) {