fix(32): WR-02 report the request locale's primary tag on record events

Admin saves copied the raw context locale into RecordSaved.Locale, which
could be a whole Accept-Language header, while reorder reported a
normalised code. Saves, child writes and reorder now report the primary tag
of the request locale. The pact doc says what Locale is and that per-locale
work keys on RecordChange.Locale, since an admin save writes the row in the
default content locale.

docs/plugins/extending.md

@@ -48,7 +48,7 @@ Each listener names the plugin that owns it, so an error or a recovered panic in
 
 The framework fires two events for every record written through the admin: `pact.RecordSaved` after a create or update, `pact.RecordDeleted` after a delete. They are fired as pointers, so a listener takes `*pact.RecordSaved` or `*pact.RecordDeleted`.
 
-A `pact.RecordSaved` carries the admin controller ID (`Controller`), the record's morph type (`Model`, the model's `MorphName`, else its table name), its primary key as text (`ID`), the request locale, `Created` and `Changes`: one `pact.RecordChange` per changed attribute with its old and new value. A translated value is reported once per locale, with `Locale` set, so a translated slug change is visible too. A save that changed nothing still fires, with no changes. A `pact.RecordDeleted` carries `Controller`, `Model` and `ID`.
+A `pact.RecordSaved` carries the admin controller ID (`Controller`), the record's morph type (`Model`, the model's `MorphName`, else its table name), its primary key as text (`ID`), the request locale (`Locale`, the primary tag of the administrator's request locale in the admin; values written to the row are the default content locale's, so key per-locale work on the changes), `Created` and `Changes`: one `pact.RecordChange` per changed attribute with its old and new value. A translated value is reported once per locale, with `Locale` set, so a translated slug change is visible too. A save that changed nothing still fires, with no changes. A `pact.RecordDeleted` carries `Controller`, `Model` and `ID`.
 
 The events fire after the write commits, never for a write that rolled back, and synchronously before the response is written. A listener's error is logged and never fails the write. Admin creates and updates, deletes and bulk deletes, tree reorders (a moved record reports its parent and sort columns) and relation-manager child writes all fire them; a child's event names the controller that edits the child's model. A form extension's `Save` receives the same changes in `pact.AdminFormSave.Changes`, inside the save transaction.