Fix empty 'On this page' menu caused by duplicate/empty heading IDs

[simonhamp]

Summary

• HeadingRenderer: Generate unique, non-empty heading IDs by tracking used IDs per conversion and suffixing duplicates (-1, -2, etc.). Empty slugs fall back to heading.
• CommonMark: Reset heading ID tracking before each convertToHtml() call so IDs stay unique within a document but don't leak across conversions.
• plugin-toc.blade.php: Defensively filter out headings with empty IDs and use a separate tocKey for Alpine's x-for :key to handle any remaining duplicates in pre-rendered HTML.

Test plan

• Added test for duplicate headings getting unique IDs
• Added test for empty slug fallback
• Added test for ID reset between conversions
• Existing heading renderer tests still pass
• Verify on production plugin page that the "On this page" menu populates correctly

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