chore(spec): regenerate the migration registry, spec-changes.json and the upgrade guide

Generator output only (gen:migration-registry, gen:spec-changes,
gen:upgrade-guide) for the stage-9 entry prose.

Claude-Session: https://claude.ai/code/session_014EJ1ED8X4MMrT18BhVx4tx
Co-authored-by: Claude <noreply@anthropic.com>

Author: claude