Hoist the two line-for-line-identical per-destination minimum-interval limiters
(discovery forward, routing error) into a shared PerAddrRateLimiter, and fold
the duplicated exponential-backoff math (discovery originator, FMP retry) into a
shared backoff_ms helper, both in a new proto/rate_limit module. The subsystem
limiters become thin delegating newtypes so should_forward/should_send and the
backoff call conventions are preserved. Behavior is byte-identical.