Janet's %j renders a non-ASCII char as raw UTF-8 bytes (\xC3\xA9) and a
control char / DEL as \xHH with no terminating semicolon — both forms
Chez's reader rejects (invalid character \ in string hex escape). Replace
the %j string encoder with chez-str-lit: UTF-8-decode each char and emit a
Chez codepoint escape \x<cp>;, keeping \n/\t/\r/\"/\\ and staying
byte-identical to %j for printable ASCII. Applied to every string-content
site (string/keyword/symbol/var-name/regex-source).
Unblocks the p{L} utf-8 corpus case; prelude parity floor 1532 -> 1534.
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