Throws ModelVersionAheadError only when the document's reader floor exceeds
DATA_MODEL_VERSION. A doc from a NEWER engine whose changes were purely additive
keeps its old (lower) floor and still reads fine — only reader-floor-raising changes block a read.
Throws ModelVersionAheadError only when the document's reader floor exceeds DATA_MODEL_VERSION. A doc from a NEWER engine whose changes were purely additive keeps its old (lower) floor and still reads fine — only reader-floor-raising changes block a read.