Adds a sibling fn that fires a caller-supplied async hook between a
transient failure and the next attempt. The existing
retry_on_transient becomes a thin wrapper over it (no-op hook), so
no call sites churn yet.
Hook contract: fires only between attempts (N-1 times for N
attempts), never after a non-transient error or after the final
attempt. Designed for TOR NEWNYM, but the signature is generic.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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