EPIPE and ECONNRESET from the offload write now raise the same
Connection_reset the inline path reports, so callers keep one error
vocabulary and need no Unix of their own.

Stream.offload_single_write performs one writev on a systhread through a
stub that releases the runtime lock, so the calling domain keeps running
fibers while the kernel copies the buffers into the socket. Semantics
match Eio.Flow.single_write, including short writes and waiting for
writability, so callers loop exactly as they do today.

Splice two flows both ways with half-close propagation, and forward a
listening address to another one connection at a time. The immediate
consumer is uniboot's remote port forwarder (localhost in front of a
cloud instance), but nothing in it is uniboot's: the tree had
unidirectional relays only, each coupled to its owner's flow types.
The listener runs as a daemon fiber so a caller's switch still ends;
a failed dial is reported and never takes the accept loop down.