Update object detector input array creation to use span

[kevchengcodes]

Summary

Update buildInputNDArray to acquire the mutable view outside the preprocessing loop

This reworks ObjectDetector.buildInputNDArray to obtain the input NDArray's mutable storage once via mutableView(...).contiguousElements before the per-image loop, then index the resulting bounds-checked MutableSpan to write each image into its [3, H, W] batch slot.

This is safer because it drops the raw UnsafeMutablePointer arithmetic and baseAddress! force-unwrap, and treats a non-contiguous buffer (where contiguousElements is nil) as an explicit error instead of silently mis-indexing. Behavior is unchanged — each image still lands in its correct batch slot with no B3H*W intermediate buffer.