docs(fr-044): plan 4, the Cube exporter (cube-model reference generator)

The plan for the cube-model reference helper: the mapping contract per
spec section 5 with a golden fixture per row, the rollup rules, name and
escaping rules, the canonical golden, and a live lane against a pinned
Cube instance. The Cube shapes were executed against Cube v1.7.43 and
Postgres 16 before writing; @spine and @default rows are provisional on
the zero-rows-and-defaults plan.

Author: dmealing