Navigation overlay: backport patterns and template part definition

[MaggieCabrera]

This PR backports core PHP changes from Gutenberg that remove the overlay experiment and migrate navigation overlay patterns to use the core namespace.

This backport includes the following core changes:

1. Navigation Pattern Category: Adds the navigation block pattern category registration in block-patterns.php.

2. Navigation Overlay Patterns: Adds five new navigation overlay block patterns.

3. Template Part Area Constant: Adds the WP_TEMPLATE_PART_AREA_NAVIGATION_OVERLAY constant and corresponding area definition in block-template-utils.php to support navigation overlay template parts.

This backport is based on changes from WordPress/gutenberg#74968

Testing

• Verify navigation pattern category appears in the block pattern inserter
• Verify all five navigation overlay patterns are available and render correctly
• Verify navigation overlay template parts can be created and used
• Test pattern insertion in the site editor

Trac ticket: https://core.trac.wordpress.org/ticket/64589

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[github-actions]

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