#56954 - Add 'doing_sitemap'

[masteradhoc]

This patch introduces a DOING_SITEMAP constant and wp_doing_sitemap() function, following the established pattern of DOING_CRON/wp_doing_cron() and DOING_AJAX/wp_doing_ajax().

Trac ticket: https://core.trac.wordpress.org/ticket/56954

Use of AI Tools

AI assistance: Yes
Tool(s): GitHub Copilot
Model(s): Claude Sonnet 4.6
Used for: Initial implementation scaffolding. All code was reviewed,
adjusted, and tested by me.

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