ClientAssetReader typedef
Reads a client:// resource held by the host process.
Injected rather than implemented here: the obvious implementation reaches
dart:io, which would take the whole runtime off the web. A host supplies
the reader its platform can honour — and on the web that is a real reader,
not an absence (client://cache, client://temp, and client://asset
all have browser-side homes).
Implementation
typedef ClientAssetReader = Future<Uint8List?> Function(String uri);