[BUG] `af_set_backend` is declared with the wrong parameter width

[`src/core/backend.rs:7`](https://github.com/arrayfire/arrayfire-rust/blob/master/src/core/backend.rs#L7):

```rust
extern "C" {
fn af_set_backend(bknd: u8) -> c_int;
```

The C signature (`include/af/backend.h:24`) is:

```c
AFAPI af_err af_set_backend(const af_backend bknd);
```

`af_backend` is a C enum, i.e. 4-byte `int` under both MSVC and GCC. Declaring the parameter as `u8` is an ABI type mismatch.

In practice it works, because rustc emits `zeroext` for the `u8` argument and the callee reads a clean value from the full register — I checked the IR (`declare noundef i32 @af_set_backend(i8 noundef zeroext)`). So this is as a latent correctness issue rather than an active bug, but it should still be `c_uint` to match the C ABI.

The three neighbouring declarations in the same `extern` block already use `c_uint`/`c_int` correctly.

---

Found by Claude Opus 5.

從 src/core/backend.rs:7 開始，將 af_set_backend 與同一 extern 區塊中的相鄰宣告進行比較。檢查 include/af/backend.h:24 以確認 C enum 型別，然後更新 Rust 宣告使其與 ABI 相符，接著執行儲存庫的 Rust 檢查或測試。當該宣告使用正確的參數寬度且檢查通過時，即表示完成。

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