grpcerr/v1.3.0

Dynamic retry pushback (ROADMAP §5). Requires core v1.3.0.

• Changed The RetryInfo detail now prefers the error's own delay
  (errtrail.WithRetryDelay — dynamic pushback) over the code's registered
  static RetryAfter delay; errors without a per-error delay keep the
  registry behavior exactly. A delay below a WithoutPublic barrier is not
  exposed, while the registry delay — code configuration, not error data —
  still applies. Pinned end-to-end over bufconn: the dynamic value reaches
  the client's RetryDelay reader. Requires core ≥ v1.3.0.