When the previous frame's stdout.write has not drained (the outer terminal
parser is overwhelmed by a wide CR+LF burst — CJK + ANSI tool output on a
high-context session), the renderer kept writing a new frame every tick. That
piled writes onto an already-backed-up pipe and kept the macrotask queue hot,
starving the stdin 'readable' callback — the observed stdin freeze where the
agent loop keeps running but keystrokes/Ctrl-C are dead.
onRender now coalesces: while pendingWriteStart is non-null (prior write's
drain callback hasn't fired) it skips the frame and retries on the drain tick
instead of writing. A MAX_COALESCED_BACKPRESSURE_FRAMES ceiling forces a write
through after N skips so a terminal whose drain callback never fires (OSError
EIO on flush) self-heals once the pipe recovers rather than wedging forever.
TTY-only; piped stdout has no flow control. Coalesce counter resets on every
real write.
This is the stdout-backpressure strand left open after #54046 fixed the
swallowed-exception strand.
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