docs: clarify convergence semantics and delete vs export scope

- Delete: one restriction, propagated downstream only, OR at convergence
- Export: downstream + upstream context, AND at convergence
- Removed over-engineered "multiple restriction sets" abstraction
- Clarified alias nodes (same parent, multiple FKs) vs convergence (different parents)
- Non-downstream tables: excluded for delete, included for export

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: dimitri-yatsenko