PR Review: fix(ups): formalize subjects. Overall clean, well-scoped refactor. Issues: (1) No Subject impl for owned String, only for &str and &String. (2) publish_with_backoff takes &impl Subject then immediately calls .as_cow() - cleaner to just take &str. (3) MsgSubject::root() allocates via Cow::Owned on every call - could use Cow::Borrowed if M::subject() returned static str. Minor: Helper struct comment should be a complete sentence per style guide. Positives: spawn error propagation fix in message_async is correct, MsgSubject clone is zero-alloc, subject_cow computed once before chunk loop is good.