Add impl Add for Euros

Needed by the input-bound tests in the next commit, which construct an amount
one cent above the maximum.

Deliberately untested and deliberately not checked: this addition wraps in
release builds. That is acceptable while the only caller is a test, but it has
to be settled before the amortisation schedule starts summing instalments,
since a wrapped total is a wrong number that looks plausible.

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

crates/immo-core/src/euros.rs

@@ -1,4 +1,4 @@
-use std::fmt;
+use std::{fmt, ops::Add};
 
 use rust_decimal::{Decimal, prelude::ToPrimitive};
 use thiserror::Error;
@@ -48,6 +48,16 @@ impl fmt::Display for Euros {
     }
 }
 
+impl Add for Euros {
+    type Output = Self;
+
+    fn add(self, rhs: Self) -> Self::Output {
+        Euros {
+            value_as_cents: self.value_as_cents + rhs.value_as_cents,
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;