Review spot: the two restart paths added in the previous commit bumped
group.Generation and reset each member's State to Pending but left
member.Assignment populated with the prior generation's partitions.
The non-leader SyncGroup path only returns REBALANCE_IN_PROGRESS when
`member.Assignment` is empty (handleSyncGroup ~line 982). Leaving the
stale assignment in place means a member rejoining at the new
generation — before the leader's SyncGroup has published fresh
assignments — falls through that guard and is served its old
partitions from the pre-rebalance state.
Clear m.Assignment alongside m.State in both restart sites so the
guard fires and the member correctly re-enters the join/sync cycle.
Verified with a fresh-broker TestConsumerGroups run: 50.99s total,
MultipleConsumersJoin 15.25s, all four consumers each get exactly one
partition.