`complete_lines` splits on `\n` only, which is right -- this stream is
JSONL, and a record terminated by a bare `\r` would not be a record --
but the doc comment said why the remainder is held without saying what
decides where a line ends.
Worth the sentence because of what the failure would look like if the
CLI ever wrote such a line: the session goes quiet, the process is
healthy, nothing errors, and the cause is a line splitter. The
dev-updater session hit exactly this shape today reading cargo's
progress line, which is `\r`-terminated for redrawing in place, and
lost a whole build's worth of output to it.
iris
1629e0911e