Assert Street Level expansion timeout and document provider precedence

src/app/layers/streetLevel.js

@@ -5,7 +5,11 @@ import * as picking from '../../data/pickRegistry.js';
 import * as input from '../../data/inputOwnership.js';
 import * as render from '../../renderGovernor.js';
 
-/** Inject providers, or use Mapillary only when its source is supplied. */
+/**
+ * Inject providers, or use Mapillary only when its source is supplied.
+ * An explicit provider list replaces the default; unused sources.mapillary is
+ * neither constructed nor validated, including when the list is empty.
+ */
 export function createApplicationStreetLevel({
   surface,
   sources = {},