v10.13 - Folder-scoped generic parent and single-parent authority

Summary

• allow design/design.md and changelog/changelog.md when the current folder already fully scopes one chain
• require exactly one active parent model per chain so generic and semantic parents do not coexist as active owners
• preserve the released P102 chain-shape taxonomy, P103 evidence-layer separation, and P104 bootstrap/shard timing discipline while validating the active 18-rule runtime set