Add the summary tree nodes

The tree stores each built node (l, i) as one JSON line, {l, i, text,
size}, in tree/YYYY-MM-DD.jsonl, and loads all lines again at start.
Node (l, i) covers the 2^l messages from i*2^l on. The tree refuses a
node that is already built and a parent without both children, so each
node is built once and the tree never holds a gap.

A source that fits in 512 bytes is its own node with no model call. A
short message becomes the leaf "kind: text" word for word, and two short
siblings join with a newline into their parent.

Issue: beb9a70