feat: add cast() for conditional dtype conversion without unnecessary copy - #26

This PR adds a cast() method that conditionally converts array dtypes, returning the same instance when no conversion is needed.

Motivation and Context

astype() always returns a fresh copy, even when the array is already the target dtype. This is correct NumPy semantics, but it forces unnecessary allocations in callers that just want to ensure a certain dtype without paying for a copy when one isn't needed. Patterns like $arr->astype(DType::Float64) inside hot loops waste memory and time when the array already matches.

What's Changed

• Added NDArray::cast(DType $dtype): self — returns $this when dtype matches, delegates to astype() when conversion is needed
• Added cast() global function alias in Functions.php
• Documented cast() alongside astype() in the data-types and views-vs-copies guide pages, highlighting the semantic difference

Breaking Changes

None.