Claude review: rust: types: Add Ownable/Owned types

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

Patch Review

This patch introduces the core `Ownable` trait and `Owned<T>` type in a new `rust/kernel/owned.rs` file, and re-exports them from `types.rs`.

The design is clean. A few observations:

> +///         let result = NonNull::new(KBox::into_raw(result))
> +///             .expect("Raw pointer to newly allocation KBox is null, this should never happen.");

As Alice noted on the list, `KBox::into_raw` cannot return null (it returns `*mut T` from a valid allocation). A `KBox::into_raw_nonnull()` would be cleaner. This is fixed in patch 5 where the example switches to `.ok_or(ENOMEM)?`, but `ok_or(ENOMEM)` is also somewhat misleading since the null case is unreachable.

> +///    let foo = Foo::new().expect("Failed to allocate a Foo. This shouldn't happen");

Also fixed in patch 5 to use `?`.

> +/// # Invariants
> +///
> +/// - The [`Owned<T>`] has exclusive access to the instance of `T`.
> +/// - The instance of `T` will stay alive at least as long as the [`Owned<T>`] is alive.

Alice's suggestion to make the invariants more concrete (permission to call `release` once, permission for mutable access until `release`, value is pinned) would strengthen the safety reasoning throughout the series. In particular, the pinning guarantee is currently argued through "what APIs we expose" rather than as a type invariant, which makes each `Pin::new_unchecked` safety argument fragile.

> +impl<T: Ownable + Unpin> DerefMut for Owned<T> {
> +    fn deref_mut(&mut self) -> &mut Self::Target {
> +        // SAFETY: The type invariants guarantee that the object is valid, and that we can safely
> +        // return a mutable reference to it.
> +        unsafe { self.ptr.as_mut() }

As Alice noted, if "pinned" were a type invariant, this safety comment would naturally need to explain why unpinning is okay (because `T: Unpin`). Without the invariant, the safety reasoning is incomplete.

No bugs found in this patch; the issues are documentation quality.

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