Per-event stdout writes in --mode json (and text) were fire-and-forget
and relied on an empty-write flush barrier before dispose/exit. The
barrier awaited its own callback, not the preceding large write, so a
big final agent_end could be truncated when the process exited before
the pipe drained -- while still exiting 0.
Serialize every print-mode stdout write on its own completion callback
(honoring backpressure) and block shutdown on the tail so the terminal
record is delivered in full.
Fixes#7635
can1357