Constant MAX_QUIC_VARINT 

const MAX_QUIC_VARINT: u64 = _; // 4_611_686_018_427_387_903u64

The largest value a QUIC application error code can carry, 2^62 - 1.

Checked by NamespaceStream::cancel before either half of the stream is touched, so an unrepresentable code cannot half-cancel a subscription.