Handles iterative SNMP walks for one or many targets.

Single-target walks still run synchronously in the caller, while multi-target walks can share one caller process and one UDP socket without spawning a task per target.

:timeout (per PDU), :retries (re-sends of a timed-out PDU) and :walk_timeout (cap on the whole walk) follow the model described in SnmpKit.SnmpMgr.Config; a timed-out step is re-sent under a fresh request id while retries and walk time remain. Both entry points return {:error, :services_not_started} per walk when the manager services are not running.