Support `belongs_to` with the legacy `HasOne` field type

`belongs_to` now accepts `HasOne<E>` (legacy) in addition to the recommended
`BelongsTo<E>` / `BelongsTo<Option<E>>`, so entities written against the earlier
API keep compiling. A `HasOne`-typed `belongs_to` generates the pre-`BelongsTo`
`ActiveHasOne` write path — it sets this row's own foreign key on save, with
relation-keyed disambiguation for duplicate targets (two `belongs_to` to the
same entity). `BelongsTo` remains the recommended type.

Also fix `action()` skipping the insert of a brand-new row that has no column
explicitly `Set` (e.g. an auto-increment-only primary key), which left the
generated key unset and failed the subsequent model conversion. A genuinely new
row (no primary key set) now always inserts, while an unchanged row that already
has a primary key is still left untouched.