2.0.14: searchable-select: allowCustom prop; plugin scaffold page-type stub

Add an allowCustom prop to <x-gp247::searchable-select> (multi-mode): commit a
typed value not in options as a new tag (Enter key or the "+ …" row in the empty
dropdown), and preserve custom tokens when seeding on edit. Used by the
LayoutBlock page-scope field so an admin can target a page-type not in the
registry. Also add an optional page-type registration stub to the plugin
scaffold Provider.php (parallel to the sitemap block).

Modification 20260729T054157, ADR front-admin_layout-page-enum-catalog.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>