… lazily (apache#23883)

Replace GroupSpecImpl's inverted target assignment with a BitSet of the
assigned partitions of each topic, computed from the members' current
assignments on the first call to isPartitionAssigned. Only the uniform
homogeneous assignor calls isPartitionAssigned, so other assignors no
longer compute it.

This makes build in TargetAssignmentBuilderBenchmark ~40% faster for
homogeneous groups, because isPartitionAssigned no longer looks up
partitions in the inverted target assignment's TimelineHashMaps, which
are slower than regular maps.

It also fixes a bug where the assignor saw the partitions of members
removed in the same request as still assigned. The bug is not reachable
today, since we do not compute a new target assignment when a member
leaves, only on the next heartbeat, and static member replacement keeps
the replaced member's partitions assigned.

Remove ModernGroup's inverted target assignment, since it is no longer
read. For share groups, it was never correct, because a partition can be
assigned to several members of a share group, while the inverted target
assignment holds a single member per partition.

Rename buildAssignment in TargetAssignmentBuilderBenchmark to
buildGroupSpecAndAssignment, which builds a new group spec on every
invocation, so that the cost of computing the assigned partitions is
included, and replace buildGroupSpecWithoutOffload and
buildGroupSpecWithOffload with an assignorOffload parameter.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Reviewers: David Jacot <dajac@apache.org>