parse_string scanned for `"` and `\` but accepted every other byte,
including raw control characters. RFC 8259 §7 requires those bytes to be
escaped inside a string; an unescaped one is invalid JSON and must surface
a parse error, not be silently accepted.
Add `BadControlChar` to JsonParseError and reject any unescaped byte < 0x20
in the string body scan (which gates the decode path too, so escaped forms
like \t/\n/ still decode correctly; 0x20 and 0x7F are not over-rejected).
Regression test in examples/0714: raw 0x09/0x0A/0x00 each raise
BadControlChar via `?`/`!`; a positive case proves the escaped forms still
decode to the right bytes. All prior assertions kept.
2871342c0a