backoff: optional multiplicative delay growth

The package doc argued flat-is-deliberate, but two of the three services run growing envelopes — entire-api naks with 1s doubling to a 300s cap, entiredb's repoops with 5s doubling to 30s — so the shared policy could not express what the fleet actually does. Factor and MaxDelay add the growth (Nth delivery naks with NakDelay×Factor^(N-1), capped); the zero value keeps the flat envelope bit-for-bit, and uncapped growth saturates rather than wrapping into a negative delay. DelayFor is exported so callers can log the delay they are about to apply.