Omit minus sign when FormCalc Format() rounds to zero

Found while testing crates/js/src/formcalc.rs:2328-2330 ("format" number-picture branch, `if num < 0.0 { text = format!("-{text}") }`)

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

crates/js/src/formcalc.rs

@@ -2325,10 +2325,11 @@ impl Interp<'_> {
                     }
                     text = if frac.is_empty() { g } else { format!("{g}.{frac}") };
                 }
+                let nonzero = text.chars().any(|c| ('1'..='9').contains(&c));
                 if pic.contains('$') {
                     text = format!("${text}");
                 }
-                if num < 0.0 {
+                if num < 0.0 && nonzero {
                     text = format!("-{text}");
                 }
                 Val::Str(text)

crates/js/src/tests.rs

@@ -805,6 +805,14 @@ mod formcalc {
         o.result.unwrap_or_default()
     }
 
+    /// A negative value that rounds to zero under the picture prints without a minus sign.
+    #[test]
+    fn format_does_not_print_negative_zero() {
+        assert_eq!(calc(r#"Format("9.9", -0.04)"#), "0.0");
+        assert_eq!(calc(r#"Format("99", -0.2)"#), "00");
+        assert_eq!(calc(r#"Format("9.9", -1.5)"#), "-1.5");
+    }
+
     /// Assigning one long value to many fields at once stops at the run's byte budget and is
     /// charged per copy, and a reset list names each field once.
     #[test]