Add a Policy.OnRetry observability hook

Right now the only way to observe individual retry attempts from outside this package is to reimplement retry logic around it, or to infer attempt counts after the fact from a Result/Job's attempt history. Teams embedding pkg/retry in a different service (this package is explicitly designed to be go get-able standalone) routinely want per-attempt visibility -- custom metrics with their own labels, a structured log line their own logging pipeline expects, a trace span per attempt -- without forking the retry loop itself. Without a hook, every consumer either duplicates the loop or goes without that visibility.

Policy has no way to observe individual attempts as they happen; a caller only ever learns the final outcome (Do) or the full history after the fact (DoWithResult). A field, OnRetry func(attempt int, err error, delay time.Duration), has been added as scaffolding in pkg/retry/policy.go, but nothing calls it yet.

Configure a Policy with OnRetry set to a callback that records each call. Run retry.DoWithResult against an operation that fails twice then succeeds. Observe: OnRetry is never invoked, even though two attempts failed and were retried.

This is exactly the kind of small, additive extension point a general-purpose library needs so downstream consumers can integrate their own observability without becoming a contributor to (or a fork of) this package.

DoWithResult's attempt loop in pkg/retry/do.go already computes delay right before sleeping it (via nextDelay), in the branch that runs only when the attempt failed, was retryable, and wasn't the last attempt. That's precisely the one place OnRetry needs to be invoked from -- guard for policy.OnRetry != nil first, since it's optional.