fix: make topk() return numerically-keyed tuple for consistency - #30

This PR changes topk() from returning string-keyed arrays (['values' => ..., 'indices' => ...]) to numerically-keyed arrays ([$values, $indices]) to match the convention used by every other multi-NDArray return in the library.

Motivation and Context

topk() was the only method returning a string-keyed tuple (['values' => NDArray, 'indices' => NDArray]). All other multi-NDArray returns — svd(), qr(), eig(), eigh(), lstsq(), split(), vsplit(), hsplit(), meshgrid() — use numerical positional keys. This inconsistency meant callers had to remember different destructuring patterns depending on the method. The new signature matches NumPy and PyTorch semantics where destructuring is natural: [$values, $indices] = $arr->topk(3).

What's Changed

• topk() and its private helpers (topkAxisOp, topkFlatOp) now return [NDArray, NDArray] instead of ['values' => NDArray, 'indices' => NDArray]
• Updated PHPDoc @return annotations from array{values: NDArray, indices: NDArray} to array{0: NDArray, 1: NDArray}
• Updated all tests to use positional destructuring ($topk[0]/$topk[1] instead of $topk['values']/$topk['indices'])
• Updated API documentation examples

Breaking Changes

Yes. Anyone relying on $result['values'] or $result['indices'] to access topk results will need to update to $result[0]/$result[1] or destructure with [$values, $indices] = $arr->topk(...).