backup_render_before_rebuild() copies render/fulltext.md (and
render-map.json, heading-events.json) to versions/v{N}/ before
rebuild overwrites them. Creates/updates versions/manifest.json
with version metadata. Idempotent: skips when no render exists.
Hook placed between phase 3 and phase 4 in _rebuild_one_paper.
6 new tests.
