Exponential backoff with jitter

Design the retry delays for a flaky API:

1. backoffSchedule(retries, base, cap) → array of delays: base × 2^attempt, each capped at cap e.g. backoffSchedule(5, 100, 1000) → [100, 200, 400, 800, 1000]
2. withJitter(schedule, rand) → full jitter: each delay becomes rand() × delay (rand injected for testability; use Math.floor on the result)