feat(通知模板): 支持缺字段按空值渲染与空行省略

Motivation:
统一 crypto-strategy 与 trade-signal 两类交易信号的模板字段，使同一套通知模板可复用；字段缺失或为空时不再导致渲染报错或输出空行，通知内容更整洁。

Changes:

* 渲染器缺失字段改为按空值处理，新增 line 函数实现空值整行省略
* crypto-strategy 信号补充止盈价、止损价、平均价、收益额等模板字段
* trade-signal 信号保留原始报文中的额外字段以适配统一模板
* 修复结构化日志将 error 作为值直接输出导致的格式问题

internal/engine/renderer_test.go

@@ -18,10 +18,41 @@ func TestRenderer(t *testing.T) {
 	}
 }
 
-func TestRenderer_Error(t *testing.T) {
+func TestRendererMissingKeyEmpty(t *testing.T) {
 	r := NewRenderer()
-	_, err := r.Render("{{.nonexistent}}", map[string]interface{}{})
-	if err == nil {
-		t.Error("expected error for missing field, got nil")
+	_, err := r.Render("x{{.nonexistent}}y", map[string]interface{}{})
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestRendererLineOmitsEmpty(t *testing.T) {
+	r := NewRenderer()
+	tmpl := `{{line "币种" .symbol}}{{line "平均价" .totalAvgPx}}{{line "价格" .price}}{{line "止损价" .stopLossPrice}}`
+	out, err := r.Render(tmpl, map[string]interface{}{
+		"symbol":        "ICP",
+		"price":         2.273,
+		"totalAvgPx":    "",
+		"stopLossPrice": 0,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !strings.Contains(out, "币种：ICP") || !strings.Contains(out, "价格：2.273") {
+		t.Fatalf("out=%q", out)
+	}
+	if strings.Contains(out, "平均价") || strings.Contains(out, "止损价") {
+		t.Fatalf("empty lines should be omitted, out=%q", out)
+	}
+}
+
+func TestRendererLineMissingKey(t *testing.T) {
+	r := NewRenderer()
+	out, err := r.Render(`{{line "平均价" .totalAvgPx}}{{line "币种" .symbol}}`, map[string]interface{}{"symbol": "ICP"})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if strings.Contains(out, "平均价") || !strings.Contains(out, "币种：ICP") {
+		t.Fatalf("out=%q", out)
 	}
 }