feat: add scalar support to minimum() and maximum() - #32

This PR adds scalar argument support to minimum() and maximum(), matching the API of add(), multiply(), and other element-wise operations.

Motivation and Context

minimum() and maximum() only accepted NDArray arguments, forcing callers to use awkward workarounds like $arr->clamp($value, INF) just to get a per-element floor. Every other element-wise operation (add, subtract, multiply, divide) already supported scalar arguments so this was an inconsistent gap.

What's Changed

• minimum() and maximum() now accept Complex|float|int|NDArray — when a scalar is passed, each array element is compared against that value
• Two new Rust FFI functions (ndarray_minimum_scalar, ndarray_maximum_scalar) handle all numeric dtypes via element-wise mapping
• Added 5 test cases covering scalar float, scalar int, negative values, 2D arrays, and dtype preservation
• Updated API documentation with scalar examples

Breaking Changes

None. The NDArray path is unchanged. The new scalar path returns the same dtype as the input array.