run-length plus base64 distance-to-mate, little-endian pairs per position
canonical id: K + extras strongest-first + K, e.g. KQK, KQBK
K
KQK
KQBK
indexed positions: 64^pieces times the side to move
run-length plus base64 WDL bytes, one per position
run-length plus base64 distance-to-mate, little-endian pairs per position