Claude review: rust: Add `OwnableRefCounted`

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

This trait enables the `Owned<T> <-> ARef<T>` conversion, analogous to `UniqueArc <-> Arc`. The design is correct.

**Doc link issue** in `aref.rs`:
```rust
/// [`OwnableRefCounted`] allows to convert between unique and
/// shared references (i.e. [`Owned<Self>`](crate::types::Owned) and
/// [`ARef<Self>`](crate::types::Owned)).
```
The last link `[`ARef<Self>`](crate::types::Owned)` points to `Owned` instead of `ARef`. This should link to `crate::sync::aref::ARef` or simply use `[`ARef<Self>`]`.

The `OwnableRefCounted::into_shared` default implementation correctly reinterprets the `Owned` pointer as an `ARef` pointer — this works because the refcount is 1 for `Owned`, which is a valid state for `ARef` as well. The safety reasoning is that the `Owned` already "owns" one refcount increment.

The `TryFrom<ARef<T>> for Owned<T>` is a good ergonomic addition.

The `From<Owned<T>> for ARef<T>` impl is also clean and delegates to `into_shared`.

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Generated by Claude Code Patch Reviewer