`sendReply` ignored `proto.write`'s `false` return, so a program resolving
several large values in one `asyncio.gather` round encoded every reply in the
same turn and queued all of them in fd 3's writable buffer. Binding resolution
carries no seam-level byte cap to bound that, and the failure kills the host
process rather than failing the run: measured on a 64 KiB-highWaterMark pipe,
eight 4 MiB replies buffered 32.0 MiB at once against 0.0 MiB once paced.
Replies now go through a queue that encodes and writes one frame at a time,
awaiting `drain` when the pipe is full. The encode happens inside the loop, so a
queued reply the run no longer needs is dropped by the `settled` check without
ever being serialized.
This was previously deferred on the grounds that serializing would narrow the
seam's concurrency contract. That reasoning was wrong: the child matches each
reply to its `call` by id from a pump that reads fd 3 continuously, so arrival
order was never observable, and the bindings still run concurrently. Only the
host's peak memory and the flush timing change. The README entry recording the
deferral is removed and the Agent Note records the mechanism instead.
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