[Fix] fuzzy_storage: peer-pipe write resume and shutdown drain

[Fix] fuzzy_storage: peer-pipe write resume and shutdown drain

fuzzy_peer_try_send retried short writes from byte 0 of the command
instead of resuming at the offset already sent, so a partial write
followed by a watcher-driven retry shoved garbage into the peer pipe.

Track the bytes sent on the request and resume from there.  Convert
the helper to a tri-state (DONE / AGAIN / FATAL) so the watcher can
keep firing on transient short writes and only stop+free on completion
or a hard error.

Also link pending requests into a list on the ctx so worker shutdown
can drain any whose write watcher never fires (e.g. on non-update
workers where the event loop has already broken out), instead of
leaking the up_req allocations.