Half-way to separate treatment of sources and targets.
After source/target split: sources==targets case working again.
More headway towards source/target split.
Rename: box_ids -> srcntgt_box_ids.
Minor improvement to source/target index finder.
Remove current box ID from scan state.
Work on terminology in tree build. Eliminate 'unsplit_box_id'.
Change order of ops in tree build: Prune first--less churn that way.