Function surb_resolution_wait 

pub(crate) fn surb_resolution_wait(configured: Option<Duration>) -> Duration

The wait a node should use, given what its configuration says.

Unset means the default; zero is honoured as “do not wait”, which drops a return packet the first time its SURBs are missing. Kept as one function so both meanings live in one place rather than being re-derived at the call site.