Claude review: rust: rename `AlwaysRefCounted` to `RefCounted`.

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

Patch Review

This is a large mechanical rename across 20 files, splitting the old `AlwaysRefCounted` into `RefCounted` (the trait with `inc_ref`/`dec_ref`) and a new marker `AlwaysRefCounted: RefCounted` that permits `&T -> ARef<T>`.

> +/// Implementers must ensure that no safety invariants are violated by upgrading an `&T` to an
> +/// [`ARef<T>`]. In particular that implies [`AlwaysRefCounted`] and [`crate::types::Ownable`]
> +/// cannot be implemented for the same type, as this would allow violating the uniqueness guarantee
> +/// of [`crate::types::Owned<T>`] by dereferencing it into an `&T` and obtaining an [`ARef`] from
> +/// that.
> +pub unsafe trait AlwaysRefCounted: RefCounted {}

The safety documentation here is clear and the rationale for the split is well explained. The mechanical changes across all the driver files look correct -- each existing `AlwaysRefCounted` impl is split into `RefCounted` (with the actual `inc_ref`/`dec_ref`) plus an empty `AlwaysRefCounted` marker.

> +// SAFETY: We currently do not implement `Ownable`, thus it is okay to obtain an `ARef<Request>`
> +// from a `&Request` (but this will change in the future).
> +unsafe impl<T: Operations> AlwaysRefCounted for Request<T> {}

The "but this will change in the future" note is a good heads-up for downstream work.

No issues found in this patch.

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