refactor: use nginx -s reload instead of systemd

- doc: add comment

internal/hostnginx/config_test.go

@@ -6,6 +6,7 @@ import (
 	"testing"
 )
 
+// serverConfiguration 含受管理 upstream 块、8080 被注释、8081 活动的示例配置，供各测试共享。
 const serverConfiguration = `http {
     upstream yms-server {
         # yms-update managed upstream begin
@@ -16,6 +17,8 @@ const serverConfiguration = `http {
 }
 `
 
+// TestActiveBackendPortReadsExactManagedBlock 验证 ActiveBackendPort 能从示例配置中
+// 精确读取到活动端口 8081。
 func TestActiveBackendPortReadsExactManagedBlock(t *testing.T) {
 	port, err := ActiveBackendPort([]byte(serverConfiguration))
 	if err != nil || port != 8081 {
@@ -23,6 +26,8 @@ func TestActiveBackendPortReadsExactManagedBlock(t *testing.T) {
 	}
 }
 
+// TestRenderBackendPortPreservesConfigurationAndSwitchesOneServer 验证 RenderBackendPort
+// 只切换一条服务器行，其余配置保持不变，且渲染结果的活动端口正确。
 func TestRenderBackendPortPreservesConfigurationAndSwitchesOneServer(t *testing.T) {
 	rendered, err := RenderBackendPort([]byte(serverConfiguration), 8080)
 	if err != nil {
@@ -38,6 +43,8 @@ func TestRenderBackendPortPreservesConfigurationAndSwitchesOneServer(t *testing.
 	}
 }
 
+// TestManagedBlockRejectsAmbiguousOrAlteredInput 验证受管理块在端口双活、端口缺失、
+// 选项被篡改或出现多余行等情况下都会被拒绝。
 func TestManagedBlockRejectsAmbiguousOrAlteredInput(t *testing.T) {
 	tests := map[string]string{
 		"both active":    strings.Replace(serverConfiguration, "# server 10.11.1.117:8080", "server 10.11.1.117:8080", 1),