[FIX] Implement backlinks tracking for atomic highlights parent files

[coderabbitai]

Description

When processing atomic highlights, we need to implement a backlinks tracking mechanism to maintain references between parent files and their atomized highlights.

Context

In the processAtomicHighlights method (src/services/deduplicating-vault-writer.ts:377), there's currently a placeholder for backlinks implementation:

// FIXME: Implement this for Backlinks
// Make sure we keep track of the "parent" file we've written
// through a special field based on `_primaryFile`

Implementation Notes

• Track the parent file relationship through a special field based on _primaryFile
• Ensure bidirectional linking between parent notes and their atomic highlight children
• Consider how this integrates with the existing tracking properties and frontmatter

References

• PR: Atomizer release  #80
• Requested by: @johannrichard