Buffer pre-dial writes on deferred sockets — the queued agent request's head no longer drops

- a socket whose dial hasn't started (the agent's maxSockets queue, the caller-lookup wait) has no fd but is logically connecting: write bytes now buffer and flush at establishment instead of silently dropping, which left the queued exchange (and the program) hanging
- all five differential fixtures (2573-2577) match Node byte-for-byte