A rename: its mappings, always written (empty included), each {from, to}
so no column name becomes an object key (PLAN §10.3). Names are kept
exactly, untrimmed, for validation to judge. A key on a mapping that this
version doesn't know could change the rows, so it keeps the node as an
Unknown node, once every known field has been read (PLAN §11.4).
A rename: its mappings, always written (empty included), each
{from, to}so no column name becomes an object key (PLAN §10.3). Names are kept exactly, untrimmed, for validation to judge. A key on a mapping that this version doesn't know could change the rows, so it keeps the node as an Unknown node, once every known field has been read (PLAN §11.4).