split_blocks was tested on the shapes it was written against. These are
the ones a real reply contains -- a fence with blank lines in it, a `---`
inside a fence, a nested list, a fence directly under a heading, a table,
a quote -- plus the property RowBlocks::apply_delta actually depends on,
checked at every character boundary of a message that has all of them:
growing a message may rewrite its last block and never an earlier one, or
common_prefix must say so. No defect found; the split already held.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
irisandClaude Fable 5.1
a56a928b0c