Fold a single document's source through a plan, producing one merged update.
Each step's migrator for kind is applied in plan order; every non-empty
result is merged into the accumulating update, so later steps win on colliding
keys. Returns an empty object when nothing changed — the caller skips the
document.update() entirely in that case.
Fold a single document's source through a plan, producing one merged update.
Each step's migrator for
kindis applied in plan order; every non-empty result is merged into the accumulating update, so later steps win on colliding keys. Returns an empty object when nothing changed — the caller skips thedocument.update()entirely in that case.