Fix DType::from rejecting S16, U16 and F16 - #391

Fixes #386.

DType::from(u32) asserted t <= DType::U64 as u32 (9), but the enum extends to F16 = 12:

U64 = 9, S16 = 10, U16 = 11, F16 = 12,

So the conversion panicked for three of the crate's own supported types. Since Array::get_type() calls DType::from, any f16/s16/u16 array panicked on get_type() (and on Debug, which calls it):

let a = randu::<half::f16>(dim4!(3, 3));
let t = a.get_type();   // panicked before this fix

The fix bounds the assert at F16 instead of U64.

Tests

Added dtype_from_u32_roundtrip in core::defines::tests, asserting every discriminant from F32 through F16 round-trips through DType::from.

Verification

The crate links the native ArrayFire library, which isn't installable on this CI-less machine, so I linked the test binary against a stub libaf.so exporting the referenced af_* symbols (this bug is pure Rust logic and never calls into the native library at runtime).

• Before the fix: cargo test --lib dtype_from_u32_roundtrip fails — assertion failed: DType::F32 as u32 <= t && t <= DType::U64 as u32 at util.rs:78 (panics at t = 10).
• After the fix: test core::defines::tests::dtype_from_u32_roundtrip ... ok.