Claude review: rust: clk: implement Send and Sync

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

Patch Review

> +    // SAFETY: It is safe to call `clk_put` on another thread than where `clk_get` was called.
> +    unsafe impl Send for Clk {}
> +
> +    // SAFETY: It is safe to call any combination of the `&self` methods in parallel, as the
> +    // methods are synchronized internally.
> +    unsafe impl Sync for Clk {}

The safety comments are accurate. The `Clk` type wraps a `*mut bindings::clk`, which prevents auto-derivation of `Send`/`Sync`. The C clock framework's `clk_prepare`/`clk_enable`/`clk_disable`/`clk_unprepare`/`clk_get_rate`/`clk_set_rate` operations all use internal locking (prepare_lock mutex and enable_lock spinlock), making concurrent `&self` access safe.

One minor observation: `OptionalClk` is defined as `pub struct OptionalClk(Clk)` and has no explicit `Send`/`Sync` impls. It will correctly auto-derive both traits from `Clk` after this patch. This is fine and requires no changes.

No issues found.

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