fix: reserve min buffer for high mem load control

[zmstone]

Reserve a minimum producer buffer under high memory pressure.
When drop_if_highmem is enabled and the system reports high memory usage,
the producer used to drop every newly-arrived call's bytes regardless of how
little was already buffered, which could leave the sender with nothing to
push to Kafka. The producer now keeps at least
(max_send_ahead + 1) * max_batch_bytes bytes buffered (enough to fill
every in-flight slot), and only drops the bytes that exceed this reserve.