stdout and stderr are independent pipes; a chunk from one can arrive
between two halves of a line from the other. The single shared
pending-buffer glued those fragments into garbled lines: a sheared
(fail) line went uncounted (defeating the exit-0-with-failures
backstop) and a sheared terminal summary read as truncation.
Counters stay shared; line assembly is now per-stream, and both
runners tag the stream origin. Also drops the dead ChildProcess
type import left by the killProcessGroup move.
Garry Tan
661b3d943f