Fix snippet heading anchors always scrolling to first file for non-Markdown markup

What does this MR do and why?

Fixes Heading anchors in a snippet always scroll to f... (#604237) for Non-Markdown markup. Manually verified in GDK-in-a-box.

Non-Markdown markup only

Applies to Org-mode, reStructuredText, MediaWiki, etc. Markdown is out of scope. Repository file rendering is unaffected.

Approach

Prefix heading slugs with the snippet's filename so that anchors no longer collide across files:

File (e.g.)	Before: Both scroll to first heading	After: Each scrolls to its own
planning.org	<h1 id="user-content-summary"><a href="#summary">	<h1 id="user-content-planning-summary"><a href="#planning-summary">
review.org	<h1 id="user-content-summary"><a href="#summary"> ← duplicate!	<h1 id="user-content-review-summary"><a href="#review-summary">

Notes

No docs updates included – this area isn't well covered by existing documentation. If there are specific docs that should be updated, please flag them.

MR acceptance checklist

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