refactor: separate earthquake source and display ownership

src/layers/earthquakes/source.js

@@ -0,0 +1,20 @@
+import { normalizeEarthquakeSnapshot } from './model.js';
+const API_URL =
+  'https://earthquake.usgs.gov/earthquakes/feed/v1.0/summary/all_day.geojson';
+/** Request and validate a complete USGS snapshot before it can replace displayed events. */
+export function createUsgsEarthquakeSource({
+  fetchImpl = (...args) => globalThis.fetch(...args),
+} = {}) {
+  return {
+    async getSnapshot({ signal } = {}) {
+      signal?.throwIfAborted();
+      const response = await fetchImpl(API_URL, { signal });
+      if (!response.ok) throw new Error(`USGS HTTP ${response.status}`);
+      const payload = await response.json();
+      signal?.throwIfAborted();
+      const rows = normalizeEarthquakeSnapshot(payload);
+      if (!rows) throw new Error('Malformed USGS response');
+      return rows;
+    },
+  };
+}