fix: 修复天象事件 API 在事件边界附近的重复返回问题

- 修正合月、合日、留等Last/Next/Closest接口在精确命中和事件后秒级查询时仍返回当前事件的问题，避免应用侧枚举卡死
- 收紧事件边界判定，并补强水星、金星、火星近事件路径的稳定化，保证公开API的单调前进语义
- 补充公开wrapper、跨世纪样本和外部基线回归，覆盖合月邻近事件与边界场景

moon/conjunction_test.go

@@ -80,3 +80,24 @@ func TestInvalidConjunctionPlanetReturnsZeroTime(t *testing.T) {
 		}
 	}
 }
+
+func TestNextConjunctionAdvancesPastReturnedEvent(t *testing.T) {
+	cursor := time.Date(2026, 5, 1, 0, 0, 0, 0, time.UTC)
+	first := NextConjunctionWithPlanet(cursor, ConjunctionMercury)
+	query := first.Add(time.Second)
+	next := NextConjunctionWithPlanet(query, ConjunctionMercury)
+
+	if !next.After(query) {
+		t.Fatalf("expected next conjunction after query: query=%s next=%s delta=%v",
+			query.Format(time.RFC3339Nano),
+			next.Format(time.RFC3339Nano),
+			next.Sub(query),
+		)
+	}
+	if next.Equal(first) {
+		t.Fatalf("expected next conjunction to advance: first=%s next=%s",
+			first.Format(time.RFC3339Nano),
+			next.Format(time.RFC3339Nano),
+		)
+	}
+}