feat(render): add Buf variants for JSON/GOB/WANF/HTML

Add buffered rendering methods that encode to a buffer first, then
write the response. This allows returning a proper 500 status code
if encoding fails, unlike the streaming variants which must write
the status code before encoding (an inherent HTTP constraint).

New methods:
- JSONBuf(code int, obj any)
- GOBBuf(code int, obj any)
- WANFBuf(code int, obj any)
- HTMLBuf(code int, name string, obj any)

Trade-off: one extra memory allocation per call in exchange for
correct error status codes on encoding failure.

context.go

@@ -417,6 +417,21 @@ func (c *Context) JSON(code int, obj any) {
 	}
 }
 
+// JSONBuf 先将 JSON 编码到 buffer, 成功后再写入状态码和响应体.
+// 与 JSON 相比, 编码失败时可以正确返回 500 状态码, 代价是多一次内存分配.
+func (c *Context) JSONBuf(code int, obj any) {
+	data, err := json.Marshal(obj)
+	if err != nil {
+		c.AddError(fmt.Errorf("failed to marshal JSON: %w", err))
+		c.Errorf("failed to marshal JSON: %s", err)
+		c.ErrorUseHandle(http.StatusInternalServerError, fmt.Errorf("failed to marshal JSON: %w", err))
+		return
+	}
+	c.Writer.Header().Set("Content-Type", "application/json; charset=utf-8")
+	c.Writer.WriteHeader(code)
+	c.Writer.Write(data)
+}
+
 // GOB 向响应写入GOB数据
 // 设置 Content-Type 为 application/octet-stream
 func (c *Context) GOB(code int, obj any) {
@@ -431,6 +446,20 @@ func (c *Context) GOB(code int, obj any) {
 	}
 }
 
+// GOBBuf 先将 GOB 编码到 buffer, 成功后再写入状态码和响应体.
+func (c *Context) GOBBuf(code int, obj any) {
+	var buf bytes.Buffer
+	encoder := gob.NewEncoder(&buf)
+	if err := encoder.Encode(obj); err != nil {
+		c.AddError(fmt.Errorf("failed to encode GOB: %w", err))
+		c.ErrorUseHandle(http.StatusInternalServerError, fmt.Errorf("failed to encode GOB: %w", err))
+		return
+	}
+	c.Writer.Header().Set("Content-Type", "application/octet-stream")
+	c.Writer.WriteHeader(code)
+	c.Writer.Write(buf.Bytes())
+}
+
 // WANF向响应写入WANF数据
 // 设置 application/vnd.wjqserver.wanf; charset=utf-8
 func (c *Context) WANF(code int, obj any) {
@@ -445,6 +474,19 @@ func (c *Context) WANF(code int, obj any) {
 	}
 }
 
+// WANFBuf 先将 WANF 编码到 buffer, 成功后再写入状态码和响应体.
+func (c *Context) WANFBuf(code int, obj any) {
+	data, err := wanf.Marshal(obj)
+	if err != nil {
+		c.AddError(fmt.Errorf("failed to encode WANF: %w", err))
+		c.ErrorUseHandle(http.StatusInternalServerError, fmt.Errorf("failed to encode WANF: %w", err))
+		return
+	}
+	c.Writer.Header().Set("Content-Type", "application/vnd.wjqserver.wanf; charset=utf-8")
+	c.Writer.WriteHeader(code)
+	c.Writer.Write(data)
+}
+
 // HTML 渲染 HTML 模板
 // 如果 Engine 配置了 HTMLRender，则使用它进行渲染
 // 否则，会进行简单的字符串输出
@@ -469,6 +511,29 @@ func (c *Context) HTML(code int, name string, obj any) {
 	c.Writer.Write(fmt.Appendf(nil, "<!-- HTML rendered for %s -->\n<pre>%v</pre>", name, obj))
 }
 
+// HTMLBuf 先将 HTML 模板渲染到 buffer, 成功后再写入状态码和响应体.
+func (c *Context) HTMLBuf(code int, name string, obj any) {
+	if c.engine != nil && c.engine.HTMLRender != nil {
+		if tpl, ok := c.engine.HTMLRender.(*template.Template); ok {
+			var buf bytes.Buffer
+			err := tpl.ExecuteTemplate(&buf, name, obj)
+			if err != nil {
+				c.AddError(fmt.Errorf("failed to render HTML template '%s': %w", name, err))
+				c.ErrorUseHandle(http.StatusInternalServerError, fmt.Errorf("failed to render HTML template '%s': %w", name, err))
+				return
+			}
+			c.Writer.Header().Set("Content-Type", "text/html; charset=utf-8")
+			c.Writer.WriteHeader(code)
+			c.Writer.Write(buf.Bytes())
+			return
+		}
+	}
+	// 默认简单输出
+	c.Writer.Header().Set("Content-Type", "text/html; charset=utf-8")
+	c.Writer.WriteHeader(code)
+	c.Writer.Write(fmt.Appendf(nil, "<!-- HTML rendered for %s -->\n<pre>%v</pre>", name, obj))
+}
+
 // Redirect 执行 HTTP 重定向
 // code 应为 3xx 状态码 (如 http.StatusMovedPermanently, http.StatusFound)
 func (c *Context) Redirect(code int, location string) {