docs: consolidate restricted diagram design documents

Merge restricted-diagram.md and restricted-diagram-spec.md into a single
document reflecting the final implementation: _restrict_freetable for SQL
generation, OR/AND convergence semantics, data-driven part_integrity
post-check, and dry_run support.

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

Author: dimitri-yatsenko