Briefing for the loop that builds the host-side servicer for persist/* IO ops,
making lib/persist's durable backend actually durable. Points at the Blocker
spec in plans/persist-on-sx.md as the authoritative contract; hard rules on
build isolation (worktree _build only, never clobber the shared binary) and not
pkilling the shared sx_server.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
