* worker: don't leak task goroutines on forced shutdown
Stop() drains in-flight tasks for 30s, then terminates the manager loop.
A task still running past that deadline later reports completion through
w.cmds - getAdmin in completeTask, the ActionIncTask* send, removeTask -
but with the loop gone and cmds unbuffered, those sends and the response
reads behind getAdmin/getTaskLoad block forever, leaking the goroutine.
Close a done channel when the loop exits and route the task-goroutine
sends through it so they abort and return zero values instead of
blocking. getAdmin can now return nil mid-shutdown, so collapse its
double-call sites to a single nil-checked call to avoid a deref.
* worker: abort remaining manager-loop sends after shutdown
Extend the post-shutdown abort to the sends that still blocked: Stop()'s
own ActionStop (so a second Stop, e.g. an admin-shutdown timer racing an
explicit one, doesn't hang), setTask, and handleTaskCancellation. Route
them through w.done so they return instead of blocking when the loop is
gone. Stop is now idempotent.
