D020 - Unterminated text is a separate value variant, not a flag

0x0004 is UTF-8 without a terminator and 0x0204 is UTF-8 with one. The first version held both as Value::Text and derived the format from the variant, so every string was written back terminated.

That turned a PS3 save's unterminated field into a terminated one: a file one byte longer, differing in a format code fifty-five bytes in. Found only because real files were compared byte for byte rather than parsed and eyeballed.

Two variants rather than one plus a flag, so a value cannot be held next to a format it disagrees with. The format is derived from the value and the derivation is now total.