What

Introduce a new template part type for Navigation overlays, allowing users to create and design custom mobile overlay layouts within the Editor. Overlays will be composed of standard blocks and saved as reusable template parts, enabling full visual control over their content and styling.

This work does not include the functionality for the Navigation block to select or assign a custom overlay - that will be handled in #73080.

Why

Making overlays template parts gives both users and theme authors flexibility to define bespoke mobile navigation experiences. Themes will be able to register and provide overlay templates, while the focused editing mode for template parts ensures an intuitive, distraction-free environment for designing overlays.

How

Add support for a new “Overlay” template part type that can be created, selected, and edited directly in the block editor. This will form the foundation for customisable mobile navigation, enabling users and themes to define unique overlay designs using standard block-editing tools.

By default the overlay will be full screen height/width and the template editing experience should reflect this.

Overlay design should prioritise stylistic changes being made when editing the overlay template itself and not via custom controls on the Navigation block itself. This may involved a block-based approach for elements such as the background and "close" (X) button to allow styling to rely on usual affordances for blocks.