The midpoint split index for a string chunk, backed off DOWN to a UTF-8 char boundary for text (a boundary byte is any non-continuation byte, `b & 0xc0 != 0x80`). Bytes split at the raw midpoint. Minted strings are ASCII, so the backoff is a no-op there, but the mutator must not assume it.
(data: &[u8], is_text: bool)
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