Keep and name a stored value its form widget can no longer show #2649

Part of #2426 (F-FRM). Found by the independent review of #2638. The gap predates it.

A stored value that its widget can't show is dropped without a word when an unrelated field is edited.

• An enum value "green" stored before the type's values became [red, blue] has no matching option, so the select posts an empty value. An optional field is then left out of the new version.
• The same happens after a field changes from text to number, or from date-time to date-only.
• The only sign of the loss is in the change set's diff.

Scope

• The form shows a stored value its widget can't hold next to the field, and says that it no longer fits the field's type.
• An untouched field with such a value keeps it on save, so it is the person's choice to clear or replace it. A required field says it needs a value the type takes.
• Tests: a title-only edit keeps an enum value no longer listed and a text in a field now a number, and the form names both.

Acceptance

• A title-only edit never drops another field's stored value without a message.