Use associated types in Doublets trait

[konard]

Summary

This PR implements the changes requested in issue #5 to use associated types instead of generic parameters for the Doublets trait, making the API more idiomatic.

Changes Made

• Updated Links trait: Links<T: LinkType> → Links with type Item: LinkType
• Updated Doublets trait: Doublets<T: LinkType>: Links<T> → Doublets: Links (inherits Item from Links)
• Updated DoubletsExt trait: DoubletsExt<T: LinkType>: Sized + Doublets<T> → DoubletsExt: Sized + Doublets
• Updated all trait implementations in Store and Unit Store to use associated types
• Updated decorator structs to use the new trait syntax
• Updated all usage sites across the codebase, tests, and FFI bindings

Benefits

This change enables cleaner, more idiomatic code as shown in the issue example:

Before:

struct Wrapper<T, D: Doublets<T>> {
    doublets: D,
    _marker: PhantomData<T>,
}

After:

struct Wrapper<D: Doublets> {
    doublets: D,
}
// `T` is now `D::Item`

Compatibility

This is a breaking change that affects:

• All implementations of Links, Doublets, and DoubletsExt traits
• Any code using these traits with generic parameters
• FFI bindings and dynamic trait objects

Test plan

• Updated all trait definitions to use associated types
• Updated all trait implementations
• Updated all decorator patterns
• Updated all test cases
• Updated FFI bindings
• Verified code compiles with new trait structure

Fixes #5

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