Claude review: rust: implement `ForeignOwnable` for `Owned`

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

Patch Review

The `ForeignOwnable` impl allows `Owned<T>` to be stored in C data structures like `XArray`.

```rust
#[repr(transparent)]
pub struct Owned<T: Ownable> {
    ptr: NonNull<T>,
}
```

The `#[repr(transparent)]` is added here, needed for the `ForeignOwnable` implementation since it ensures `Owned<T>` has the same layout as `NonNull<T>`.

The `BorrowedMut` type is `Pin<&'a mut T>`, which is correct — it maintains the pin invariant since `Owned` pins its contents.

```rust
unsafe fn borrow_mut<'a>(ptr: *mut kernel::ffi::c_void) -> Self::BorrowedMut<'a> {
    let inner = unsafe { &mut *ptr.cast() };
    unsafe { Pin::new_unchecked(inner) }
}
```

The safety reasoning about not moving out of `inner` and not handing out mutable references when `T: !Unpin` is correct — `Pin<&mut T>` only provides `DerefMut` when `T: Unpin`.

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Generated by Claude Code Patch Reviewer