Implement Links trait for Doublets instead of trait inheritance

[konard]

🎯 Summary

This PR implements the solution for issue #6 by reversing the trait relationship between Links and Doublets traits.

📋 Issue Reference

Fixes #6

🔄 Changes Made

1. Trait Hierarchy Refactoring

• Before: pub trait Doublets<T: LinkType>: Links<T>
• After: pub trait Doublets<T: LinkType>: Send + Sync

2. Core Methods Integration

Moved all core Links trait methods into the Doublets trait:

• constants(&self) -> &LinksConstants<T>
• count_links(&self, query: &[T]) -> T
• create_links(...), each_links(...), update_links(...), delete_links(...)
• get_link(&self, index: T) -> Option<Link<T>>

3. Blanket Implementation

Added automatic Links implementation for any type implementing Doublets:

impl<T: LinkType, D: Doublets<T> + ?Sized> Links<T> for D {
    // Delegates all methods to the Doublets implementation
}

4. Store Implementation Update

• Before: Implemented both Links<T> and Doublets<T> for Store
• After: Only implements Doublets<T>, automatically gets Links<T> via blanket impl

🎉 Benefits

1. Better Error Handling: As mentioned in the issue, this addresses the error handling improvements in the data-rs repository
2. Cleaner Architecture: impl Links for Doublets instead of Doublets for Links
3. Reduced Boilerplate: Automatic Links implementation for all Doublets types
4. Backward Compatibility: All existing APIs continue to work unchanged

🧪 Testing

The implementation maintains full API compatibility. All existing code using either Links or Doublets traits will continue to work without changes.

🔗 Related

• Issue: Use Links trait from data #6
• Data repository fix: linksplatform/data-rs@a2cadfa

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