feat: 新增富友支付 SDK（RSA 签名、GBK 编解码、XML 协议、回调验签）

- pkg/fuiou/types.go: WxPreCreateRequest/Response、NotifyRequest 等 XML 结构体
- pkg/fuiou/client.go: Client 结构体、NewClient、字典序+GBK+MD5+RSA 签名/验签、HTTP 请求
- pkg/fuiou/wxprecreate.go: WxPreCreate 方法，支持公众号 JSAPI（JSAPI）和小程序（LETPAY）
- pkg/fuiou/notify.go: VerifyNotify（GBK→UTF-8+XML 解析+RSA 验签）、BuildNotifyResponse

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

pkg/fuiou/wxprecreate.go

@@ -0,0 +1,56 @@
+package fuiou
+
+import (
+	"fmt"
+
+	"go.uber.org/zap"
+)
+
+// WxPreCreate 微信预下单（公众号JSAPI + 小程序）
+// tradeType: "JSAPI"（公众号）或 "LETPAY"（小程序）
+// subAppid: 公众号或小程序 AppID
+// subOpenid: 用户在对应应用下的 OpenID
+func (c *Client) WxPreCreate(orderNo, amount, goodsDesc, termIP, tradeType, subAppid, subOpenid string) (*WxPreCreateResponse, error) {
+	req := &WxPreCreateRequest{
+		Version:      "1.0",
+		InsCd:        c.InsCd,
+		MchntCd:      c.MchntCd,
+		TermId:       c.TermId,
+		RandomStr:    generateRandomStr(),
+		MchntOrderNo: orderNo,
+		TradeType:    tradeType,
+		OrderAmt:     amount,
+		GoodsDesc:    goodsDesc,
+		TermIp:       termIP,
+		NotifyUrl:    c.NotifyURL,
+		SubAppid:     subAppid,
+		SubOpenid:    subOpenid,
+	}
+
+	sign, err := c.Sign(req)
+	if err != nil {
+		return nil, fmt.Errorf("签名失败: %w", err)
+	}
+	req.Sign = sign
+
+	var resp WxPreCreateResponse
+	if err := c.DoRequest("/wxPreCreate", req, &resp); err != nil {
+		return nil, fmt.Errorf("请求富友失败: %w", err)
+	}
+
+	if resp.ResultCode != "000000" {
+		c.logger.Error("富友预下单失败",
+			zap.String("order_no", orderNo),
+			zap.String("result_code", resp.ResultCode),
+			zap.String("result_msg", resp.ResultMsg),
+		)
+		return nil, fmt.Errorf("富友预下单失败: %s", resp.ResultMsg)
+	}
+
+	c.logger.Info("富友预下单成功",
+		zap.String("order_no", orderNo),
+		zap.String("trade_type", tradeType),
+	)
+
+	return &resp, nil
+}