Fix trip stats new-country counting

tests/test_trip_stats.py

@@ -0,0 +1,29 @@
+"""Tests for trip stats."""
+
+from datetime import date
+
+import agenda
+from agenda.stats import calculate_yearly_stats
+from agenda.types import Trip
+
+
+def make_trip(start: date, country_code: str) -> Trip:
+    """Build a trip with a single country visit."""
+    return Trip(start=start, accommodation=[{"country": country_code}])
+
+
+def test_new_country_only_first_year() -> None:
+    """Ensure new countries are only counted on their first visit year."""
+    trips = [
+        make_trip(date(2024, 5, 1), "CZ"),
+        make_trip(date(2026, 6, 1), "CZ"),
+    ]
+
+    yearly_stats = calculate_yearly_stats(trips)
+    czechia = agenda.get_country("CZ")
+    assert czechia is not None
+
+    assert czechia in yearly_stats[2024]["countries"]
+    assert czechia in yearly_stats[2024]["new_countries"]
+    assert czechia in yearly_stats[2026]["countries"]
+    assert "new_countries" not in yearly_stats[2026]