Stop reading a connection at a short read

After answering, a connection went back to recv for its next request and
got EAGAIN nearly every time: a failed syscall a request. A recv that
returns less than it asked for has emptied the socket, and the
edge-triggered epoll reports the next bytes when they come (epoll(7); as
nginx does), so the connection now waits for that; a peer that hung up
is read to its end as before.

Small keep-alive GETs, 64 connections, five rounds in rotation
(Raspberry Pi 5): 131.9k/s before this step, 139.9k after. With the
three steps before it (a burst's keys in one look, the clock once a
round, the head without printf): 116.1k -> 139.9k/s, p99 672 -> 512 us,
above the 127.0k of 2026-09-26's build that the app model had since
slowed. App reads: the total 111.5k -> 125.4k/s, a 20-row page 86.2k ->
94.3k; 1 MiB reads and durable posts unchanged.

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