Single RemoteError with a merge-extensible, domain-prefixed code map;
owners throw at the failure point; streams surface marked failures;
clients consume ctx.remote directly with isRemoteFailure as the only
discrimination point and construct no failure instances.
Render reconstructable system prompts at each request-series boundary, preserve series declarations through pre-step wrappers, and keep the presentation and replay snapshots aligned across clients.
Tianyi Cui