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/engine.mjs

@@ -217,5 +217,43 @@ console.log('— save / load integrity —');
   ok(c2.stats.powerCap >= 6000, 'power recomputed on load');
 }
 
+// ---------------- tornado disasters ----------------
+{
+  const c = new City(777, 'Tornadoville');
+  for (let x = 8; x < 40; x++) c.placeStruct(STRUCT.ROAD, x, 30);
+  const tiles = [];
+  for (let x = 9; x < 30; x++) for (let z = 27; z <= 33; z++) tiles.push([x, z]);
+  c.placeZone(ZONE.RES, tiles);
+  const cp = findSpot(c, 2, 2, 12, 36);
+  c.placeStruct(STRUCT.COAL, cp[0], cp[1]);
+  for (let i = 0; i < 10; i++) c.tick();
+  const devBefore = [...c.grid.level].filter(v => v > 0).length;
+  ok(devBefore > 0, 'town exists before tornado');
+
+  let eventFired = false;
+  c.on('disaster', (e) => { if (e.type === 'tornado' && e.destroyed > 0) eventFired = true; });
+  c.spawnTornado();
+
+  const rubbleAfter = [...c.grid.rubble].filter(v => v > 0).length;
+  ok(eventFired, 'disaster event emitted with damage count');
+  ok(rubbleAfter > 0, `tornado left rubble behind (${rubbleAfter} tiles)`);
+  let anchorBad = 0;
+  for (let i = 0; i < c.grid.n; i++) {
+    const s = c.grid.struct[i];
+    if (!s || s === STRUCT.ROAD || c.grid.anchor[i] !== i) continue;
+    if (c.grid.struct[c.grid.anchor[i]] !== s) anchorBad++;
+  }
+  ok(anchorBad === 0, 'anchors consistent after tornado destruction');
+
+  // disabled flag suppresses natural spawning
+  c.disastersEnabled = false;
+  let spawnedWhileOff = false;
+  const origSpawn = c.spawnTornado.bind(c);
+  c.spawnTornado = () => { spawnedWhileOff = true; return origSpawn(); };
+  c.monthIndex = 60;
+  for (let m = 0; m < 80; m++) c.tick();
+  ok(!spawnedWhileOff, 'disabled flag prevents tornado spawns');
+}
+
 console.log(`\n${pass} passed, ${fail} failed`);
 process.exit(fail ? 1 : 0);

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;