refactor(lua): 提取常量并优化结构体字段布局

- api_var.go: 提取 argPrefix 常量，消除魔法字符串
- coroutine.go: 按逻辑分组重排 LuaCoroutine 字段，改善可读性

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/lua/api_var.go

@@ -8,6 +8,9 @@ import (
 	glua "github.com/yuin/gopher-lua"
 )
 
+// argPrefix 是 arg_ 变量的前缀，用于获取查询参数
+const argPrefix = "arg_"
+
 // ngxVarAPI ngx.var API 实现
 type ngxVarAPI struct {
 	// 请求上下文
@@ -166,12 +169,12 @@ func (api *ngxVarAPI) getVariableLua(name string) glua.LValue {
 		return glua.LString(string(api.ctx.Host()))
 
 	// URI 参数
-	case "arg_":
+	case argPrefix:
 		// 获取所有参数
 		return glua.LString(string(api.ctx.URI().QueryString()))
 	default:
 		// 检查是否是 arg_ 开头的参数
-		if len(name) > 4 && name[:4] == "arg_" {
+		if len(name) > len(argPrefix) && name[:len(argPrefix)] == argPrefix {
 			paramName := name[4:]
 			return glua.LString(string(api.ctx.QueryArgs().Peek(paramName)))
 		}
@@ -244,10 +247,10 @@ func (api *ngxVarAPI) getVariable(name string) string {
 		return ""
 	case "server_name":
 		return string(api.ctx.Host())
-	case "arg_":
+	case argPrefix:
 		return string(api.ctx.URI().QueryString())
 	default:
-		if len(name) > 4 && name[:4] == "arg_" {
+		if len(name) > len(argPrefix) && name[:len(argPrefix)] == argPrefix {
 			paramName := name[4:]
 			return string(api.ctx.QueryArgs().Peek(paramName))
 		}

internal/lua/coroutine.go

@@ -63,19 +63,17 @@ func (p Phase) String() string {
 type LuaCoroutine struct {
 	CreatedAt        time.Time
 	ExecutionContext context.Context
-	Engine           *LuaEngine
-	Co               *glua.LState
-	Cancel           context.CancelFunc
+	ngxReqAPI        *ngxReqAPI
 	RequestCtx       *fasthttp.RequestCtx
+	Co               *glua.LState
+	ngxVarAPI        *ngxVarAPI
+	ngxRespAPI       *ngxRespAPI
+	ngxLogAPI        *ngxLogAPI
+	Cancel           context.CancelFunc
 	executionCancel  context.CancelFunc
+	Engine           *LuaEngine
 	OutputBuffer     []byte
 	Exited           bool
-
-	// ngx API 实例（用于测试和 Go 层访问）
-	ngxVarAPI  *ngxVarAPI
-	ngxReqAPI  *ngxReqAPI
-	ngxRespAPI *ngxRespAPI
-	ngxLogAPI  *ngxLogAPI
 }
 
 // SetupSandbox 创建 per-request _ENV 沙箱