read-string fell through to the literal-char branch for \u and \x,
silently stripping the backslash ("A".length returned 5 instead
of 1). Added js-hex-value helper and two cond clauses that read the
hex digits via js-peek + js-hex-digit?, compute the code point, and
emit it via char-from-code. Invalid escapes fall through to the
literal-char behaviour. built-ins/String (with --restart-every 1):
65/99 → 68/99. conformance.sh: 148/148.