Visual canvas: connect specific attributes (columns), not just whole entities #5

The problem

Summary

In the visual canvas, relationships are drawn between whole entity boxes. It
is not visible which columns are actually joined. With more than one
relationship between two entities, or with composite keys, the diagram no
longer says anything useful.

What you have in mind

Expected behaviour

• Each relationship line is anchored to the concrete attribute rows on both
  sides (orders.customer_id -> customer.customer_id), not to the entity
  header.
• Multiple relationships between the same pair of entities are drawn as
  separate lines, each on its own attribute row.
• Composite keys are drawn as one relationship covering the whole column set
  (or as grouped lines), not as unrelated single lines.
• Hovering an attribute highlights the relationships it takes part in and dims
  the rest; hovering a line highlights both endpoint attributes.
• Ideally: drawing a line from one attribute to another creates the
  relationship in the model (drag from column to column).

Actual behaviour

Lines connect entity boxes only. The attribute pairing has to be looked up in
the YAML files.

Notes

• Depends on attribute-level relationship metadata in the model — see the
  primary key / foreign key issue (role: foreign_key +
  references: {entity, attribute}). Without that metadata there is nothing
  to anchor the lines to.
• For entities with many attributes it helps to keep the referenced attributes
  visible (pin key/FK columns to the top) when the box is collapsed, so the
  endpoints stay meaningful.

Context

No response