Declaratively generate diamond FK constraint views and triggers

Given tables A, B, C, and D where B and C belong to A and D belongs to C and D we want to guarantee that B and C belong to the same A.

```
A
/ \
B C
\ /
D
```

The SQL view to detect errors is something like:

```sql
create view d_consistency as
select d.id, d.b_id, d.c_id
from d
join b on d.b_id=b.id
join c on d.c_id=c.id
where b.a_id<>c.a_id;
```

Then we would need triggers on b, c, and d to check if the proposed change shows up in this view.

No files, tests, or entry points are named in the issue. First establish the project's intended declarative API for defining the A–B–C–D relationship; done means generating the consistency view and triggers for the diamond and checking proposed changes on B, C, and D.

Written by the indexing model from the issue text.