- Change sampleHasReferenceBlock to a block reading variable, which is
what it is (the distinction didn't matter previously, but will do so
when we add lacing support in full blocks because there wont be a 1:1
relationship any more.
- Move sampleRead to be a reading state variable.
- Stop abbreviating "additional"
Issue: #3026
PiperOrigin-RevId: 284000937
