settle() dropped the run from `live` eagerly, before the grace-window SIGKILL
reaped a same-group survivor. A dispose() racing a just-resolved run() then
snapshotted an empty `live` and returned while the descendant was still alive,
so teardown's "no subprocess outlives the fiber" (and its JSDoc) was false for
that window. The run now stays in `live` until the process-group poll confirms
the group empty, at which point it is both dropped from `live` and its finished
promise resolved. Adds a regression test asserting dispose() of a completed run
with a same-group survivor returns only after the survivor stops executing.
ecdb79824b