docs: clarify biz layer supports both Go struct and proto message

- Update architecture diagram to show "protocol-agnostic business logic"
- Add "Biz Layer Parameter Types" section documenting both approaches
- Update directory structure comment to reference the new section
- Sync changes for both EN and ZH documentation

Author: brooksyang

docs/en/advanced/protocol-layer.md

@@ -23,7 +23,7 @@ This document describes Bingo's pluggable protocol layer design, supporting any
                          ▼
 ┌─────────────────────────────────────────────────────────────┐
 │                        Biz Layer                            │
-│              (Proto message as params/returns)              │
+│                (Protocol-agnostic business logic)           │
 └─────────────────────────────────────────────────────────────┘
                          │
                          ▼
@@ -47,7 +47,7 @@ internal/apiserver/
 ├── biz/                    # Business logic (protocol-agnostic)
 │   ├── biz.go              # Interface definitions
 │   └── user/
-│       └── user.go         # Implementation, uses proto messages
+│       └── user.go         # Implementation (see "Biz Layer Parameter Types" section)
 │
 ├── handler/                # Protocol handlers (pluggable)
 │   ├── http/               # Standalone HTTP (Gin)
@@ -176,6 +176,42 @@ func Run(cfg *config.Config) error {
 - **Single Responsibility**: `internal/pkg/server` has no business code dependencies
 - **Reusable**: All services (apiserver, admserver, etc.) share the same infrastructure
 
+## Biz Layer Parameter Types
+
+The Biz layer method parameters and return values support two approaches:
+
+### Approach 1: Go Struct (used in this project)
+
+```go
+// pkg/api/apiserver/v1/auth.go
+type LoginRequest struct {
+    Username string `json:"username" binding:"required"`
+    Password string `json:"password" binding:"required"`
+}
+
+// internal/apiserver/biz/auth/auth.go
+func (b *authBiz) Login(ctx context.Context, req *v1.LoginRequest) (*v1.LoginResponse, error)
+```
+
+**Characteristics**:
+- Uses `binding` tags for request validation, natively supported by Gin
+- Supports native Go types (`time.Time`, `*string` for optional fields, etc.)
+- gRPC Handler requires Go Struct ↔ Proto Message conversion
+
+### Approach 2: Proto Message
+
+```go
+// Generated from proto file
+// internal/apiserver/biz/auth/auth.go
+func (b *authBiz) Login(ctx context.Context, req *pb.LoginRequest) (*pb.LoginResponse, error)
+```
+
+**Characteristics**:
+- Proto file serves as single source of truth for type definitions
+- gRPC Handler requires no type conversion
+- HTTP Handler requires JSON ↔ Proto conversion
+- Validation logic requires protoc-gen-validate or manual implementation
+
 ## Handler Implementation
 
 ### gRPC Handler

docs/zh/advanced/protocol-layer.md

@@ -23,7 +23,7 @@
                          ▼
 ┌─────────────────────────────────────────────────────────────┐
 │                       Biz 层                                 │
-│              (Proto message 作为参数/返回值)                  │
+│                    (协议无关的业务逻辑)                        │
 └─────────────────────────────────────────────────────────────┘
                          │
                          ▼
@@ -47,7 +47,7 @@ internal/apiserver/
 ├── biz/                    # 业务逻辑（协议无关）
 │   ├── biz.go              # interface 定义
 │   └── user/
-│       └── user.go         # 实现，参数用 proto message
+│       └── user.go         # 实现（参数类型见「Biz 层参数类型」章节）
 │
 ├── handler/                # 协议处理器（可插拔）
 │   ├── http/               # 独立 HTTP（Gin）
@@ -176,6 +176,42 @@ func Run(cfg *config.Config) error {
 - **单一职责**：`internal/pkg/server` 不依赖任何业务代码
 - **可复用**：所有服务（apiserver、admserver 等）共用同一套基础设施
 
+## Biz 层参数类型
+
+Biz 层的方法参数和返回值支持两种方案：
+
+### 方案一：Go Struct（本项目示例）
+
+```go
+// pkg/api/apiserver/v1/auth.go
+type LoginRequest struct {
+    Username string `json:"username" binding:"required"`
+    Password string `json:"password" binding:"required"`
+}
+
+// internal/apiserver/biz/auth/auth.go
+func (b *authBiz) Login(ctx context.Context, req *v1.LoginRequest) (*v1.LoginResponse, error)
+```
+
+**特点**：
+- 使用 `binding` tag 做请求验证，Gin 原生支持
+- 支持 Go 原生类型（`time.Time`、`*string` 可选字段等）
+- gRPC Handler 需要做 Go Struct ↔ Proto Message 转换
+
+### 方案二：Proto Message
+
+```go
+// 由 proto 文件生成
+// internal/apiserver/biz/auth/auth.go
+func (b *authBiz) Login(ctx context.Context, req *pb.LoginRequest) (*pb.LoginResponse, error)
+```
+
+**特点**：
+- Proto 文件作为唯一类型定义
+- gRPC Handler 无需类型转换
+- HTTP Handler 需要 JSON ↔ Proto 转换
+- 验证逻辑需使用 protoc-gen-validate 或手写
+
 ## Handler 实现
 
 ### gRPC Handler