feat(osi): 实现 MD5 头签名与请求头组装（T-002）

password=md5("ts=<ts>&ask=<ask>") 32 位小写；ask 仅参与签名不进 headers
测试向量与本地 Python 联调脚本对齐

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

osi/sign.go

@@ -0,0 +1,30 @@
+package osi
+
+import (
+	"crypto/md5"
+	"encoding/hex"
+)
+
+type HeaderInput struct {
+	OrgCode  string
+	DeviceSN string
+	UserName string
+	Ask      string
+	TS       string
+}
+
+func SignPassword(ts, ask string) string {
+	sum := md5.Sum([]byte("ts=" + ts + "&ask=" + ask))
+	return hex.EncodeToString(sum[:])
+}
+
+func BuildHeaders(input HeaderInput) map[string]string {
+	return map[string]string{
+		"Content-Type": "application/json",
+		"orgCode":      input.OrgCode,
+		"deviceSN":     input.DeviceSN,
+		"ts":           input.TS,
+		"userName":     input.UserName,
+		"password":     SignPassword(input.TS, input.Ask),
+	}
+}