Add legacy `HasOne` belongs_to test suite

Add `blogger_legacy`, a mirror of the `blogger` fixtures whose `belongs_to`
fields keep the legacy `HasOne<E>` type, and `belongs_to_legacy_tests` covering
nested writes (has_one + belongs_to, including a brand-new nested parent),
many-to-many through a junction, duplicate-target `belongs_to`, and the entity
loader. Document the `BelongsTo` / `HasOne` split in the changelog.