Add a Code Library snippet for older-than-source CRP results

[ajaydsouza]

Goal

Add a copy-and-paste Code Library example that restricts Contextual Related Posts results to posts published before the current source post.

Existing coverage

The existing crp-change-query-dates.php example demonstrates crp_query_date_query for same-day matching. It does not cover older-than-source behavior, which is the practical snippet-level alternative to YARPP's past_only option.

Deliverables

• Add contextual-related-posts/crp-older-posts.php or an equally clear filename under the queries category.
• Use crp_query_date_query and derive the source post publication date safely from the query arguments or current post.
• Define and document the boundary behavior so posts published after the source are excluded.
• Add a uniquely prefixed callback, PHPDoc, and matching Markdown companion under src/content/snippets/contextual-related-posts/.
• Include the timezone and timestamp-precision caveat needed for a reliable date comparison.
• Set requiresPro: false, list the correct requirements, and link the crp_query_date_query hook reference.

This should remain a focused query example; it should not add a CRP setting or attempt to implement YARPP compatibility in the plugin.