feat(32-02): record events from every admin write path

- Delete and each bulk-delete row fire RecordDeleted after commit
- tree reorder fires RecordSaved per moved record (parent/sort in model types)
- relation-manager child create/update/delete fire events naming the
  controller that edits the child's model
- TestRecordEvents covers create, update, translated slug, no-op, rollback
  (validation, FormBeforeUpdate, extension Save), failing listener, reorder,
  child writes, delete and bulk delete; extension Save sees the changes
- docs: Record events in extending.md, admin-controllers notes, cabana README

docs/backend/admin-controllers.md

@@ -222,6 +222,8 @@ A hook or scope that has to read the database during a write should use the tran
 
 Scope reads and writes with `pact.ListExtendQuery` and `pact.FormExtendQuery` rather than checking in a hook: the scope then applies to every route, including relation and action routes.
 
+Other plugins do not implement these hooks to follow your records; they listen for the record events every admin write fires after its commit: `pact.RecordSaved` from create, update, tree reorder (one per moved record) and relation-manager child create and update, `pact.RecordDeleted` from delete, each row of a bulk delete and relation-manager child delete. Record and bulk actions write through your own code and fire nothing unless that code calls `cabana.EmitRecordSaved` or `cabana.EmitRecordDeleted`. See [Record events](../plugins/extending.md#record-events).
+
 ### Form-only values and rules
 
 A form may collect values that are not columns of the record (see [Form-only fields](forms.md#form-only-fields)). The Form hooks read what the administrator submitted with `cabana.VirtualFieldsFromContext(ctx)`. The map holds only the fields that were sent and that the field's `context` allows for this operation, so a missing key means "not submitted". Values are scalars as decoded from the request: a string, a bool, a `json.Number` or nil. The map is a copy, and the second result is false outside a create or update.
@@ -606,7 +608,7 @@ 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`, and `Values`, the submitted values of the extension's fields whose `context` allows the operation. `Save` runs on every create and update, with or without values.
+- 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.
 - 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.