fix(maps): stop all delivery once the controller is destroyed

A listener could destroy the controller and re-subscribe one still pending in
the snapshot, which then ran. Delivery now stops at destruction and a destroyed
controller takes no listeners. Adds ordering tests for adds, duplicates and
re-adds during delivery, and moves the current-state note after the section
intro.

src/maps/controller.js

@@ -131,7 +131,8 @@ export class MapSourceController {
    * @returns {() => void} Unsubscribe.
    */
   subscribe(listener) {
-    if (typeof listener !== 'function') return () => {};
+    // A destroyed controller never settles again, so it takes no listeners.
+    if (typeof listener !== 'function' || this._destroyed) return () => {};
     this._subscribers.add(listener);
     return () => {
       this._subscribers.delete(listener);
@@ -142,6 +143,7 @@ export class MapSourceController {
     if (this._destroyed || !this._subscribers.size) return;
     const state = this.getState();
     for (const listener of [...this._subscribers]) {
+      if (this._destroyed) return;
       if (!this._subscribers.has(listener)) continue;
       try {
         listener(state);

src/maps/map-subscription-ownership.test.mjs

@@ -53,3 +53,37 @@ test('destroying the controller during delivery stops remaining listeners', asyn
   await controller.setStack('fixture');
   assert.deepEqual(calls, ['destroy']);
 });
+
+test('a listener re-added after destroy during delivery is not called', async () => {
+  const controller = fixture();
+  const calls = [];
+  const later = () => calls.push('later');
+  controller.subscribe(() => {
+    calls.push('destroy');
+    controller.destroy();
+    controller.subscribe(later);
+  });
+  controller.subscribe(later);
+  await controller.setStack('fixture');
+  assert.deepEqual(calls, ['destroy']);
+});
+
+test('delivery order: added during delivery waits, duplicates run once, re-added runs', async () => {
+  const controller = fixture();
+  const calls = [];
+  const twice = () => calls.push('twice');
+  const readded = () => calls.push('readded');
+  let offReadded;
+  controller.subscribe(() => {
+    calls.push('first');
+    controller.subscribe(() => calls.push('added-late'));
+    offReadded();
+    controller.subscribe(readded);
+  });
+  controller.subscribe(twice);
+  controller.subscribe(twice);
+  offReadded = controller.subscribe(readded);
+  await controller.setStack('fixture');
+  assert.deepEqual(calls, ['first', 'twice', 'readded']);
+  controller.destroy();
+});