TreeIds grow too fast when merging annotations with agglomerate skeletons

[fm3]

Agglomerate trees get the segmentId as treeId (meant to deterministically set the tree color)

Merging trees will give the new tree the old treeId + max(allTreeIds).

This combination grows the tree ids too fast in repeated merging.

We ran into #5009 but it may also become an int32 overflow issue.

Compare discussion at

https://scm.slack.com/archives/C5AKLAV0B/p1747727467745729?thread_ts=1747725772.503309&cid=C5AKLAV0B