Claude review: gpu: nova-core: use core library's CStr instead of kernel one

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

Patch Review

Changes three files to use `core::ffi::CStr` instead of `kernel::str::CStr`. In the `elf` submodule of `firmware/gsp.rs`, the `kernel::prelude::*` import is replaced with a targeted import of just `kernel::bindings` and `kernel::transmute::FromBytes`, while `CStr` and `size_of` come from `core` directly.

> +    use core::{
> +        ffi::CStr,
> +        mem::size_of, //
> +    };
> +
>      use kernel::{
>          bindings,
> -        prelude::*,
>          transmute::FromBytes, //
>      };

The removal of `prelude::*` in the `elf` submodule is fine since the only thing the submodule used from the prelude was `CStr` (the submodule doesn't use `Result`, `Vec`, or other prelude items, as confirmed by inspecting the code). `size_of` was already imported via `core::mem::size_of` in the existing code.

No issues found.

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