feat: add conditional return types for axis-dependent reduction methods - #22

This PR adds PHPStan conditional return type annotations to reduction and norm methods whose return type depends on whether $axis is null.

Motivation and Context

When calling methods like $arr->sum() with $axis = null, the return is always a scalar (Complex|float|int), but the union return type Complex|float|int|NDArray forces static analysis tools like PHPStan to require type narrowing before using the result. Conditional return types allow tools to infer the correct type based on the $axis argument value, eliminating unnecessary boilerplate assertions.

What's Changed

• Added @return ($axis is null ? <scalar> : NDArray) PHPDoc annotations to sum, mean, min, max, argmin, argmax, product, var, std in HasReductions trait
• Added @return ($axis is null ? float : NDArray) PHPDoc annotation to norm in HasLinearAlgebra trait

Breaking Changes

None.