[ot][spam][crazy] journal: doubly append-only balanced trees

[Undiscussed Horrific Abuse, One Victim of Many]

ok here's what to do, i think:

-> find the largest subtrees on both sides of a write
-> start from each subtree and walk toward the write, accumulating the
largest full subtrees possible

this collects data that isn't changed together into a single reusable
full tree to be quickly referenced in an index

this can likely replace branch merging and such