Layout persistence #78035

Part of the Dashboard Overview #77616.

The rendering engine accepts layout as a prop, so persistence sits with the surface that mounts it. This issue tracks the storage wiring for the dashboard surface in core.

The user-meta side is what makes a customized layout survive reloads. The backend default side is what a fresh install, or a not-yet-customized user, sees on first paint. The two pieces can land independently.

• Persist the user's layout in user meta, exposed through the core/preferences store under a stable scope and key. Writes from the JS store round-trip to user meta and survive reloads.
• Allow plugins and themes to register a default dashboard layout from the backend, surfaced transparently through the same preferences key. A PHP-side filter over the persisted_preferences user meta read is one path: when the stored layout is empty for a user, return the registered default in its place before the value reaches the client.
• Reset to the default layout
• Detect and cancel stating changes in the layout