修正回撤指标的初始净值基线并补充回归测试

crates/fidc-core/src/metrics.rs

@@ -558,7 +558,9 @@ fn alpha_beta(
 }
 
 fn drawdown_stats(nav: &[f64]) -> (f64, usize) {
-    let mut peak = 0.0_f64;
+    // NAV is measured against the pre-period capital. The first real loss
+    // must not become a new zero-drawdown baseline.
+    let mut peak = 1.0_f64;
     let mut max_drawdown = 0.0_f64;
     let mut duration = 0_usize;
     let mut max_duration = 0_usize;
@@ -767,6 +769,28 @@ fn safe_div(numerator: f64, denominator: f64, fallback: f64) -> f64 {
 mod tests {
     use super::*;
 
+    #[test]
+    fn drawdown_includes_initial_nav_without_adding_a_trading_day() {
+        let (drawdown, duration) = drawdown_stats(&[0.9, 0.99]);
+        assert!((drawdown + 0.1).abs() < 1e-12);
+        assert_eq!(duration, 2);
+        assert_eq!(drawdown_stats(&[1.0, 1.1, 1.1]), (0.0, 0));
+        assert_eq!(drawdown_stats(&[0.0]), (-1.0, 1));
+        assert_eq!(drawdown_stats(&[]), (0.0, 0));
+    }
+
+    #[test]
+    fn first_day_loss_is_preserved_in_shared_backtest_metrics() {
+        let curve = vec![
+            equity_point("2025-01-03", 99.16648349337, 98.81608059815, 100.0),
+            equity_point("2025-01-06", 99.68551588547, 98.65392198168, 98.81608059815),
+        ];
+        let metrics = compute_backtest_metrics(&curve, &[], &[], &[], 100.0, None).unwrap();
+        assert!((metrics.max_drawdown + 0.0083351650663).abs() < 1e-12);
+        assert_eq!(metrics.total_trade_days, 2);
+        assert_eq!(metrics.max_drawdown_duration_days, 2);
+    }
+
     fn equity_point(
         date: &str,
         total_equity: f64,