_emit() wrote to stdout synchronously from QEMU callback context — the
iothread fires _on_uart_tx per UART byte, each becoming ~45 bytes of
JSON. When the parent's pipe reader stalled, the 64 KB pipe filled and
the write blocked INSIDE the QEMU iothread, freezing the entire guest.
Observed as an intermittent (~1 in 10) boot hang: serial output stops
right after the ROM log / rtcinit line — exactly where the accumulated
boot events cross the pipe capacity — and never recovers. Direct worker
harness runs (fast reader) never reproduced it; the browser/WS path did.
Route _emit() through a bounded queue drained by a dedicated writer
thread (opportunistic batching, one write per drain). Under extreme
backpressure events are dropped and counted on stderr — losing telemetry
beats freezing the emulated CPU. The shutdown path posts a sentinel and
joins the writer so crash/system events still flush before os._exit.
Verified on staging: 10/10 UI run/stop cycles + 8/8 direct harness boots
with identical serial latency (~0.95s to first app output).
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