field type needs a constraint to enforce only one primary

Problem/Motivation

The application logic that only one item in a field can be the primary is only enforced at the widget level. It should be enforced with a constraint.

Steps to reproduce

// Create a node with 2 values, then load it and do this:
    $node->field_test_per[0]->primary = 1;
    $node->field_test_per[1]->primary = 1;
    dsm($node->validate()); // This should show a validation error, but it does not!
    $node->save();

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes