Back off plan resends and stop on refusal

Resends now double from 2 s up to 30 s, reset when an acknowledgement
arrives, and stop after five sends. An update the server refused outright
(implementation active, or an oversized update answered with a same-epoch
reset) is not repeated; the next open still replays it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>