Block Editor: Use dedicated navigation-overlay icon for template part area

[getdave]

Backports the navigation-overlay icon change from WordPress/gutenberg#75426.

In get_allowed_block_template_part_areas() in block-template-utils.php, the Navigation Overlay area previously registered with 'icon' => 'overlay' as a placeholder. This updates it to the dedicated 'navigation-overlay' icon introduced alongside the icons package change in Gutenberg.

Trac ticket: https://core.trac.wordpress.org/ticket/64629

Use of AI Tools

This pull request was prepared with assistance from Claude Code (Anthropic, claude-sonnet-4-5-20250929). The code change has been reviewed and is the responsibility of the author.

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[mikachan]

Matches the changes in WordPress/gutenberg#75426, LGTM!

[getdave]

Committed in https://core.trac.wordpress.org/changeset/61623