Adds key to _group.members and updates the nid-keyed mirror.
Returns true if the membership state changed. Caller is responsible
for bumping the generation if needed (or the coordinator's
bumpBulkGen does it as part of the broader bump).
Bumps the generation counter. Public so optimized callers can
invalidate downstream caches without going through a member mutation.
Mirrors today's _bumpBulkGen (controller-side caller would also
call coordinator.bumpAnimGen() to bump the broad counter; the
coordinator's bumpBulkGen() does both).
Creates a fresh AnimationGroup, disposing any prior one first.
initialValue is 0.0 for expandAll (forward), 1.0 for collapseAll
(reverse). Wires the constructor-injected onTick and
onStatusChanged callbacks.
Disposes the current group's controller (if any) and zeros every
member's mirror slot. Mirrors today's _disposeBulkAnimationGroup
— set the field to null FIRST to prevent the disposing controller's
final synchronous status event from interfering.