The reviewer's standing issue: line.toString('utf8') silently replaces illegal
bytes with U+FFFD, so a forged frame could land a corrupted completion value
(the honest child's lossless encoder never emits non-UTF-8, so such a frame is
hostile traffic). The fd-3 frame decode now uses a fatal UTF-8 decoder: an
illegal byte throws and the frame is dropped, same treatment as the
unsafe-integer check. A forged illegal-UTF-8 done frame is verified to be
dropped (the run settles on the program's real return), and reverting to
toString makes the case fail.
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