refactor(maps): separate source factories and scene coordination

src/maps/credits.js

@@ -0,0 +1,25 @@
+import * as Cesium from 'cesium';
+
+/** Static credits follow the source actually on screen, including fallback. */
+export function createMapCredits(viewer) {
+  let active = null;
+  let markup = null;
+  return {
+    show(html) {
+      if (html === markup) return;
+      const display = viewer?.scene?.frameState?.creditDisplay;
+      if (!display) return;
+      try {
+        if (active) display.removeStaticCredit(active);
+        active = html ? new Cesium.Credit(html, true) : null;
+        markup = html;
+        if (active) display.addStaticCredit(active);
+      } catch {
+        /* Switching remains usable on older credit displays. */
+      }
+    },
+    destroy() {
+      this.show(null);
+    },
+  };
+}