fix: correct misleading ORDER_HALF comment in bip-0374 #2059

The comment for ORDER_HALF incorrectly stated it represents "Number of valid distinct x coordinates on the curve". This is wrong - ORDER_HALF is simply half the group order, commonly used for ECDSA low-S signature normalization. The number of valid x coordinates on secp256k1 is approximately (p-1)/2 where p is the field size, which is a completely different value.

Also clarified the ORDER comment from "plus 1 for infinity" to "including infinity" for better accuracy.

Hi @zeroprooff thank you, this looks like an accurate improvement to the code comments, but the code itself has changed and has also been moved. Will look into rebasing and, if still relevant, cherry-picking your commit into a batched cleanup PR (the commit will remain credited to you).