Make catalog feed configuration optional and explicit

src/standalone/layerSources.js

@@ -22,9 +22,11 @@ import { createReferenceSources } from '../sources/reference.js';
 export { createReferenceSources as createStandaloneReferenceSources } from '../sources/reference.js';
 
 /** Select standalone providers without starting their acquisition. */
-export function createStandaloneLayerSources() {
+export function createStandaloneLayerSources(overrides = {}) {
+  if (!overrides || typeof overrides !== 'object' || Array.isArray(overrides))
+    throw new TypeError('Source overrides must be an object');
   const mapTiles = createOpenFreeMapSource();
-  return {
+  const defaults = {
     ...createReferenceSources(),
     flights: createFlightSource(),
     military: createMilitarySource(),
@@ -51,4 +53,9 @@ export function createStandaloneLayerSources() {
       token: import.meta.env?.MAPILLARY_CLIENT_TOKEN || '',
     }),
   };
+  for (const name of Object.keys(overrides)) {
+    if (!Object.hasOwn(defaults, name))
+      throw new TypeError(`Unknown source: ${name}`);
+  }
+  return { ...defaults, ...overrides };
 }