take_events kept returning Option<EventBatchView> and defaulting through the
question-mark operator, so a batch missing a field read as end of stream and the
ConsumerEvents enum added in the previous round described nothing. It returns
Batch or Unreadable now, and a test publishes a batch with no droppedBefore,
asserts it is reported as unreadable naming the field, and asserts the next real
batch still reads.
The checkpoint-envelope-v1 amendment cites the rule that lets a required
manifest field land with envelopeVersion still 1 while no production file
exists.
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