Apply proper negative lightness handling in DIN99o

We really shouldn't try to be clever here by trying to mirroring the
behavior for negative lightness. Instead, just let the algorithm as
specified handle it, and when it it breaks down (somewhere past -200)
we can just return NaN for now. In most cases, this will be more than
sufficient. Anyone hitting this limit is pushing the space well pass
reasonable limits.

coloraide/spaces/din99o.py

@@ -39,8 +39,8 @@ def lab_to_din99o(lab: Vector) -> Vector:
     """XYZ to DIN99o."""
 
     l, a, b = lab
-    val = 1 + abs(C2 * l)
-    l99o = C1 * math.copysign(1, l) * math.log(val) / KE
+    val = 1 + C2 * l
+    l99o = C1 * math.log(val) / KE if val >= 0 else math.nan
 
     if a == 0 and b == 0:
         a99o = b99o = 0.0

docs/src/markdown/about/changelog.md

@@ -8,6 +8,7 @@
 -   **FIX**: Much more accurate ICtCp matrices.
 -   **FIX**: Fix typing of deeply nested arrays in `algebra`.
 -   **FIX**: Fix issue with HCT undefined channel resolver.
+-   **FIX**: Proper handling of negative lightness for DIN99o.
 
 ## 2.13.1