Show 0.00 instead of -0.00 in Unit::readout for tiny negatives

Found while testing crates/doc/src/units.rs:163-165 (readout)

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

crates/doc/src/tests/units.rs

@@ -144,3 +144,10 @@ fn mixed_mul_div() {
     assert_eq!(Unit::Points.parse("10*2/5"), Some(4.0));
     assert_eq!(Unit::Points.parse("10*2/5 px"), Some(Unit::Pixels.to_pt(4.0)));
 }
+
+#[test]
+fn readout_has_no_negative_zero() {
+    assert_eq!(Unit::Points.readout(-0.001), "0.00 pt");
+    assert_eq!(Unit::Points.readout(-0.0), "0.00 pt");
+    assert_eq!(Unit::Points.readout(-12.5), "-12.50 pt");
+}

crates/doc/src/units.rs

@@ -161,7 +161,10 @@ impl Unit {
     /// A point value as the canvas measurement labels show it: two decimals and the suffix
     /// (`12.50 mm`).
     pub fn readout(self, pt: f64) -> String {
-        format!("{:.2} {}", self.from_pt(pt), self.suffix())
+        let s = format!("{:.2}", self.from_pt(pt));
+        // A small negative value rounds to `-0.00`; show it as `0.00`.
+        let s = if s == "-0.00" { "0.00" } else { &s };
+        format!("{} {}", s, self.suffix())
     }
 }