[BUG] `Array::get_backend()` can never return `OPENCL`

[phil-opp]

src/core/array.rs:392-404:

match (err_val, ret_val) {
    (0, 1) => Backend::CPU,
    (0, 2) => Backend::CUDA,
    (0, 3) => Backend::OPENCL,
    _ => Backend::DEFAULT,
}

af_backend is a bit-flag enum, not sequential (include/af/defines.h:414-417, v3.8.0):

AF_BACKEND_DEFAULT = 0,
AF_BACKEND_CPU     = 1,
AF_BACKEND_CUDA    = 2,
AF_BACKEND_OPENCL  = 4

There is no backend with value 3. An OpenCL-created array returns 4, falls through to the wildcard, and is reported as Backend::DEFAULT.

Fix: (0, 4) => Backend::OPENCL.

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Found by Claude Opus 5