The bit, as BitSpan.index.
Offset of the node's first character in the input: a body or card block,
a list item, a card / side / variant / footer divider, or a tag's [;
or, for the bit's end anchor, its last visible character.
Offset of the first character of the value it produced in the output
(JSON), or the < of the element it produced (HTML / XML); for the bit's
end anchor — always the bit's last — the closing } of its bit value,
or the < of its element's closing tag.
A fixed point inside a bit (PLAN-266): where a block-level source node starts in the input, and where the value it produced starts in the output. Linked scrolling uses them as extra knots between the bit starts.