public int lowestSequence() {
        return lowestSequence;
    }
This class represents the verification state of a specific producer id.
It contains a VerificationGuard that is used to uniquely identify the transaction we want to verify.
After verifying, we retain this object until we append to the log. This prevents any race conditions where the transaction
may end via a control marker before we write to the log. This mechanism is used to prevent hanging transactions.
We remove the VerificationGuard whenever we write data to the transaction or write an end marker for the transaction.

We also store the lowest seen sequence to block a higher sequence from being written in the case of the lower sequence needing retries.

Any lingering entries that are never verified are removed via the producer state entry cleanup mechanism.
storage/src/main/java/org/apache/kafka/storage/internals/log/VerificationStateEntry.java