restructure: move el compiler content into lang/

lang/tests/calendar/examples/local-date-arithmetic.el

@@ -0,0 +1,16 @@
+// local-date-arithmetic.el — LocalDate + Duration produces a LocalDate.
+// 2026-05-28 + 7 days crosses the May/June boundary, yielding 2026-06-04.
+
+fn run_test() -> String {
+    let d1: LocalDate = local_date(2026, 5, 28)
+    let week: Duration = 7.days
+    let d2: LocalDate = d1 + week
+    let y: Int = local_date_year(d2)
+    let m: Int = local_date_month(d2)
+    let day: Int = local_date_day(d2)
+    return int_to_str(y) + "-" + int_to_str(m) + "-" + int_to_str(day)
+}
+
+fn main() -> Void {
+    println(run_test())
+}