Recipes around building a form to edit existing Entities

Central has a built-in Entity editor. But in many cases it's better to have a form to make those edits so there can be constraints and appropriate widgets. This also means different individuals can see a different set of fields that they can edit.

The basic structure of those forms is to first allow looking up an Entity and then either populate the fields with defaults or show prior values in labels.

https://forum.getodk.org/t/entity-office-hours-thursday-december-12th-3-30pm-utc/51337/2?u=ln

No repository files or tests are named. Start by reviewing the built-in Entity editor and the linked forum discussion, then identify the documented form patterns for looking up an Entity, showing prior values or defaults, applying constraints and widgets, and limiting editable fields; done when those recipes are clear and actionable.

Written by the indexing model from the issue text.