Fix signum: use gt/lt comparisons instead of negate

sign(-x) - sign(x) broke for two reasons:
- Unsigned types (CBool, Word32): negate wraps (e.g. -1_u8 = 255),
  making sign(-x) = 0 for all positive inputs, so signum always returns 0
- Float zero: af_sign(-0.0) = 1 due to sign-bit check, giving signum(0.0) = 1

Replace with cast(gt x 0) - cast(lt x 0), which avoids negate entirely
and correctly handles unsigned types and IEEE 754 negative zero.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: dmjio

src/ArrayFire/Orphans.hs

@@ -39,7 +39,7 @@ instance (Num a, AFType a) => Num (Array a) where
   x + y       = A.add x y
   x * y       = A.mul x y
   abs         = A.abs
-  signum x    = A.sign (-x) - A.sign x
+  signum x    = A.cast (A.gt x 0) - A.cast (A.lt x 0)
   negate arr  = A.scalar @a (fromInteger (-1)) `A.mul` arr
   x - y       = A.sub x y
   fromInteger = A.scalar . fromIntegral