Module backoff 

Exponential backoff with jitter for hello/probe scheduling.

Used by neighbor discovery to schedule retransmits and probes without creating correlated bursts across multiple nodes (thundering-herd effect).

The algorithm:

1. Start at initial_interval.
2. On each failure, double the interval (capped at max_interval).
3. Add uniform random jitter of ±jitter_fraction of the current interval.
4. On success, reset to initial_interval.

Structs

BackoffConfig

Static configuration for a backoff strategy.

BackoffState

Per-instance mutable backoff state.