The collection's state narrowed to the instances keep admits: the index subset (the entries
as written, untouched) and, for every declared series the state carries, the SAME positions'
bytes. Everything else in the state (a timestamp, a whole-console reading) passes through
untouched. A state with no index — the unmeasured row — passes through whole: absence is a
fact and narrowing it would invent one; so does a state whose index the codec refuses. Returns
the state object itself when nothing changed, so a caller can tell.
The collection's state narrowed to the instances
keepadmits: the index subset (the entries as written, untouched) and, for every declared series the state carries, the SAME positions' bytes. Everything else in the state (a timestamp, a whole-console reading) passes through untouched. A state with no index — the unmeasured row — passes through whole: absence is a fact and narrowing it would invent one; so does a state whose index the codec refuses. Returns the state object itself when nothing changed, so a caller can tell.