Idempotent migration from the old archive/<party>/<slot> layout to the
flat top-level party-peer layout: moves the workspace/register slots
(incoming/working/staging/reviewing/mdl/rsk) out to <project>/<slot>/<party>/,
moves archive/<party>/ssr.yaml → ssr/<party>.yaml (the party registry),
synthesizes a minimal ssr/<party>.yaml for any archived party that lacks
one, and leaves archive/<party>/{received,issued} in place (the WORM
record). --dry-run flag; per-party summary; tested + idempotent.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
