Absolute byte offset in the file. Filled in by header/index.ts once
computeLayout has produced attrIndexBegin -- 0 here is a placeholder,
not a claim that the blob starts at the file's first byte.
branchingFactor
branchingFactor:number
columnIndex
columnIndex:number
length
length:number
Whole blob, INCLUDING its payload section -- not just the tree nodes.
numUniqueItems
numUniqueItems:number
Unique KEYS, not features: a key shared by many features counts once.
Absolute byte offset in the file. Filled in by header/index.ts once computeLayout has produced
attrIndexBegin-- 0 here is a placeholder, not a claim that the blob starts at the file's first byte.