feat(layers): add FAA drone restrictions airspace layer

Show US prohibited and restricted special-use airspace from the keyless
FAA SUA FeatureServer as an off-by-default Infrastructure layer, with
share-link token 3 and an informational-only disclaimer.

src/layers/droneAirspace/source.js

@@ -0,0 +1,23 @@
+import { readResponseJsonCapped } from '../../sources/httpBody.js';
+
+/** Request a normalized snapshot through the bounded, same-origin FAA proxy. */
+export function createDroneAirspaceSource({
+  fetchImpl = (...args) => globalThis.fetch(...args),
+} = {}) {
+  return {
+    async getSnapshot({ signal } = {}) {
+      signal?.throwIfAborted();
+      const response = await fetchImpl('/api/drone-airspace', { signal });
+      if (!response.ok) throw new Error(`FAA SUA HTTP ${response.status}`);
+      const payload = await readResponseJsonCapped(
+        response,
+        80 * 1024 * 1024,
+        signal,
+      );
+      signal?.throwIfAborted();
+      if (!Array.isArray(payload?.rows))
+        throw new Error('Malformed drone airspace snapshot');
+      return payload.rows;
+    },
+  };
+}