Included changes:
- Add base-interface under:
- service router-advert interface <lan-if> prefix ::/64 base-interface <wan-if>
- Render Base6Interface <wan-if>; in radvd.conf when configured.
- Add validation guard:
- base-interface is only allowed with wildcard prefix ::/64.
- Extend smoketests:
- Positive case: ::/64 + base-interface commits and renders correctly.
- Negative case: non-::/64 prefix with base-interface fails commit.