F2.2: reject raw control bytes (U+0000..U+001F) in JSON strings

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.

examples/expected/0714-modules-json-reader.stdout

@@ -33,4 +33,11 @@ err-fraction: ok
 err-leading-zero: ok
 err-overflow: ok
 err-unterminated: ok
+err-raw-tab: ok
+err-raw-lf: ok
+err-raw-nul: ok
+esc-ctrl-len: ok
+esc-tab: ok
+esc-lf: ok
+esc-u: ok
 === DONE ===