Fixed-cardinality Remove button hardcodes target_id, breaking link (and other non-entity_reference) fields

Problem/Motivation

I'm using this module on a fixed-cardinality link field (Allowed number of values: 8) and noticed that clicking Remove on a row in the middle of the list duplicates the last filled row instead of clearing it.

Steps to reproduce

1. Add a Link field to any content type.
2. Set Allowed number of values to a fixed number greater than 1 (for example, 3).
3. Create a node, fill all 3 rows with three different links (A, B, C), and save.
4. Edit the node and click Remove on row 1 (B).
5. Save the node and reopen it.

Expected: rows 1 and 2 contain A and C; row 3 is empty.
Actual: the last row repaints with C, so the saved node has A, C, C.

Digging into multiple_fields_remove_button_fixed_submit_handler(), I found that the last delta gets blanked like this:

NestedArray::setValue($formInputs, $moving_element['#parents'], [
  'target_id' => '',
  '_weight' => $cardinality - 1,
]);

That works fine if the field is an entity_reference (which has a single target_id input), but a link field has uri and title inputs, it doesn't have a target_id at all.

So when the AJAX rebuild looks for uri and title on the last delta, it finds nothing, falls back to the default value (whatever's saved on the entity), and renders that.

Proposed resolution

Instead of writing a hardcoded target_id/_weight shape, build the empty row from the actual widget.

Remaining tasks

User interface changes

API changes

Data model changes