Add zero known-answer test for Euros

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

crates/immo-core/src/euros.rs

@@ -73,4 +73,10 @@ mod tests {
             "Mantissa should be equal to i64::MAX"
         );
     }
+
+    #[test]
+    fn from_0_cents_to_decimal_stays_zero() {
+        let euros = Euros::from_cents(0);
+        assert_eq!(euros.as_decimal(), Decimal::ZERO);
+    }
 }