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null character sorts greater than 0x7f and less than 0x80
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No, it is an overlong sequence which is explicitly forbidden by the UTF-8 standard. As are any attempts at encoding any character with a code point above U+10FFFF.

BUT! Those are potentially useful encodings, and I think the SQLite library's policy of allowing arbitrary streams of bytes (that don't include NUL) to be inserted into a TEXT column is useful to many people, and to suddenly begin enforcing strict UTF-8 compliance would be a disservice to the community. Especially when people who want strict UTF-8 compliance can achieve it in other ways because of just how flexible SQLite is.