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

tests/soak.mjs

@@ -10,6 +10,7 @@ let pass = 0, fail = 0;
 const ok = (cond, name) => { if (cond) { pass++; console.log('  ✔', name); } else { fail++; console.log('  ✘ FAIL:', name); } };
 
 const c = new City(90210, 'Soakville');
+c.disastersEnabled = false; // economy focus here; tornado behaviour is covered by engine.mjs
 const g = c.grid;
 c.money = 60000;