api/server: only record distill_loss when Overall>0, exclude fallback pollution

go/internal/api/server.go

@@ -238,14 +238,17 @@ func NewServer() http.Handler {
 		// 更新仪表盘指标
 		selfoptimize.Dash.RecordUseful()
 		// 记录蒸馏质量损失（avg_distill_loss = 1 - Overall）
-		loss := 1.0 - result.Overall
-		if loss < 0 {
-			loss = 0
+		// 仅记录真实蒸馏：Overall > 0 表示 LLM 成功返回，过滤 fallback (Overall=0)
+		if result.Overall > 0 {
+			loss := 1.0 - result.Overall
+			if loss < 0 {
+				loss = 0
+			}
+			if loss > 1 {
+				loss = 1
+			}
+			selfoptimize.Dash.RecordDistillLoss(loss)
 		}
-		if loss > 1 {
-			loss = 1
-		}
-		selfoptimize.Dash.RecordDistillLoss(loss)
 	}
 
 	// ─── 跨 Agent 缓存失效回调 ───────────────────