Use two-strategy approach to work around async shortcode limitation
in deeply nested Nunjucks includes:
- blog.njk: async {% unfurl %} shortcode (top-level, works fine)
- recent-posts.njk: sync {{ url | unfurlCard | safe }} filter
(reads from pre-populated disk cache)
eleventy.before hook scans content files and pre-fetches all
interaction URLs before templates render, ensuring the sync filter
always has data — even on first build.
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