Clamp negative signals to zero in hlg_decode

Found while testing crates/color/src/transfer.rs:52

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

crates/color/src/transfer.rs

@@ -49,6 +49,7 @@ pub fn hlg_encode(e: f64) -> f64 {
 }
 
 pub fn hlg_decode(s: f64) -> f64 {
+    let s = s.max(0.0);
     if s <= 0.5 { s * s / 3.0 } else { (((s - HLG_C) / HLG_A).exp() + HLG_B) / 12.0 }
 }
 
@@ -112,6 +113,11 @@ mod tests {
         }
     }
 
+    #[test]
+    fn hlg_decode_clamps_negative() {
+        assert_eq!(hlg_decode(-0.5), 0.0);
+    }
+
     #[test]
     fn pq_hlg_roundtrip() {
         for i in 0..=50 {