fix(32): CR-01 check required extension fields on the lifted value

A required: true field in a form extension became a rule on the owner's
model, where its value never exists, so every save of the host form failed,
also for administrators who cannot see the extension. mergedRules now skips
extension fields; liftExtensionValues checks required on the submitted value
for visible extensions only (create: must be sent and not blank; update:
only when sent).

docs/backend/admin-controllers.md

@@ -609,6 +609,7 @@ Every field and tab of a compiled form has a stable name: the field key, and the
 An extension adds value fields (`text`, `textarea`, `number`, `checkbox`, `switch`, `dropdown` with listed options, `markdown`, `mltext`, `mltextarea`, `mlmarkdown` and plugin field types), `widget` fields and `partial` fields. Types that bind to the owner's model are refused at boot, `fileupload` included: an extension owns no attachments, so a plugin field type with `Upload` stores files instead.
 
 - On save, the extension's keys leave the request body before anything else reads it, so they never reach `Fill`, the owner's columns, its translation rows or its attachments. After the owner row is written (and its translations and after hooks have run), `Save` runs inside the same transaction with `pact.AdminFormSave`: the `Record` with its `ID`, `Created`, `Values`, the submitted values of the extension's fields whose `context` allows the operation, and `Changes`, what the owner's save changed (old and new values, a translated value per locale), as the save's `pact.RecordSaved` will report it after the commit. `Save` runs on every create and update, with or without values.
+- `required: true` on an extension field is checked on the submitted value, not as a rule on the owner's model, and only for administrators who see the extension: a create must send a value that is not null or blank (for a translatable field, the default locale's text), an update checks the value only when it is sent.
 - A translatable field (`mltext`, `mltextarea`, `mlmarkdown`, a translatable plugin field type) reaches `Save` as a `map[string]string` of locale to text, and `Load` returns it the same way. A plugin field type's `Decode` runs before `Save` (its notices are kept, a `pact.FormFieldError` is a 422 on the field) and its `Load` after the extension's `Load`.
 - A `Save` error rolls the whole save back, the owner's row included. `pact.AdminFormErrors` (field name to messages, phrase keys or text) answers 422 with those lines on the fields; any other error is logged and answered with the generic 500 body. `pact.AdminFormSaveResult.Notices` are save notices per field, shown on the field like a field type's notices; a notice may name an owner field.
 - On show and in the save response, `Load` returns the values by field name; keys that are not the extension's fields are dropped.