Migrate existing public numbers while preserving legacy references #81

Work

Build a deterministic dry-run collision/mapping report and migration using the approved #73 contract. Cover archived items, personal scopes, batch metadata, typed URLs and stored source references. Update only references whose target is provable; preserve revision history and ambiguous source text.

Acceptance

• Run against a representative migration copy with collisions and stale links.
• Record counts, mappings and authorized legacy links verified before/after.
• Rerun is safe; rollback and forward recovery are documented.
• No production migration until backup and restore procedure have been verified for the concrete release.

Dependencies

Numbering contract and allocator under #73. Recheck data at implementation time.