tp: assign dense tree node numbers

Tree IDs can be sparse, strings, or row IDs from a table much larger than the
query result. Arrays indexed by those IDs would scale with the ID range rather
than the tree being processed.

TreeNumberNodes assigns stable dense numbers as rows arrive and appends the
numbered node and parent columns to each batch. A parent is numbered when first
referenced, so child-first input works even when the parent row appears later.

The original columns stay in the batch. Later tree operators get compact
indices without losing the values needed in the result.