Timestamps returned from quiche::Connection::send

Is it guaranteed that consecutive calls to `quiche::Connection::send` will return timestamps (the field `at` in the returned `SendInfo`) in the non-decreasing order? When implementing pacing, it's significant information.

Start by locating quiche::Connection::send and the SendInfo definition in the Rust source and API documentation. Inspect how the at field is produced across consecutive send calls, then check existing tests for send ordering or pacing behavior. Done means the ordering guarantee is established and documented, or the issue is clarified with the observed behavior.

Written by the indexing model from the issue text.