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);