add the probe for the short write to the bench

add the probe for the short write to the bench

partial.py was written while chasing the truncated chunk but never
committed: it stayed in the scratch directory inside the container, so
the one tool that exercises the resumption after a short write against
the running daemon was not in the tree.

It opens a daemon socket directly, the way nginx does in direct http
mode, with an SO_RCVBUF small enough that the daemon cannot place the
whole first payload, waits without reading, then walks the chunked
framing and pairs the @BEGIN@/@END@ blocks.

It does not trigger the defect on its own: the room bootstrap is far
below the socket buffer, so the daemon never writes short. It is here to
check the framing against the real daemon, next to the socket pair
reproduction that did trigger it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014M1jiEq9cHdE5SE5j6vFuE