Hide orphaned comments from margins

[kylemcd]

What changed

• Render floating margin cards only for threads with both a comment body and a valid text anchor.
• Stop orphaned threads from reserving margin width in editor and reading views.
• Keep orphaned threads available in the comments sidebar.
• Add regression coverage for orphan detection and margin layout.

Why

A comment body can remain after its inline anchor markers are removed. The margin previously pinned these orphaned threads to the top of the editor, making an unrelated card appear stuck and then disappear while scrolling.

Validation

• npm run check
• npm run build
• Verified both anchored table comments in the reference note render beside their highlights, while orphan xbl0b never enters the margin DOM.
• Confirmed the reference note was unchanged during testing.