fix: 第二轮深度调试修复 - useSpeechSynthesis守卫+NPE防护+E2E测试完善

- P0: useSpeechSynthesis.ts cancel()增加isSupported守卫
- P0: iot_provider.go 添加loader nil检查防止NPE panic
- 新增CDP E2E v4测试脚本 14项全绿通过
- 生成第二轮修复报告 docs/debug/2026-05-21-round2-fixes.md

backend/ai-core/internal/subsession/iot_provider.go

@@ -113,14 +113,15 @@ func (p *IoTProvider) CreateContext(ctx context.Context, params CreateContextPar
 	}
 
 	// 加载人格配置
+	trueName := "昔涟"
 	loader, err := persona.NewLoader("")
 	if err != nil {
 		log.Printf("[iot-provider] 加载人格配置失败: %v", err)
 	}
-	personaConfig, _ := loader.Get("cyrene")
-	trueName := "昔涟"
-	if personaConfig != nil {
-		trueName = personaConfig.Identity.TrueName
+	if loader != nil {
+		if personaConfig, err := loader.Get("cyrene"); err == nil && personaConfig != nil {
+			trueName = personaConfig.Identity.TrueName
+		}
 	}
 
 	userName := params.Nickname