feat(payment): 虎皮椒聚合支付 - 官方插件与跨平台可执行文件

新增 xunhupay 官方包，支付 RPC 与宿主只保留订单和验签边界；官方支付包同时带上各系统 provider，避免把 Linux ELF 当成本机入口。

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/internal/protocol/payment_rpc.go

@@ -0,0 +1,72 @@
+package protocol
+
+import (
+	"path"
+	"strings"
+)
+
+func PaymentRPCBackendCandidates(entry, goos, goarch string) []string {
+	entry = strings.TrimSuffix(strings.TrimSpace(entry), ".exe")
+	goos = strings.TrimSpace(goos)
+	goarch = strings.TrimSpace(goarch)
+	if entry == "" || goos == "" || goarch == "" || path.Clean(entry) != entry || !strings.HasPrefix(entry, "backend/") {
+		return nil
+	}
+	base := path.Base(entry)
+	dir := path.Dir(entry)
+	tagged := entry + "-" + goos + "-" + goarch
+	nested := dir + "/" + goos + "-" + goarch + "/" + base
+	candidates := make([]string, 0, 6)
+	if goos == "windows" {
+		candidates = append(candidates, tagged+".exe", nested+".exe", entry+".exe")
+	}
+	candidates = append(candidates, tagged, nested, entry)
+	return uniqueStrings(candidates)
+}
+
+func HasAnyPaymentRPCBackend(entry string, files map[string][]byte) bool {
+	entry = strings.TrimSuffix(strings.TrimSpace(entry), ".exe")
+	if entry == "" {
+		return false
+	}
+	if _, ok := files[entry]; ok {
+		return true
+	}
+	if _, ok := files[entry+".exe"]; ok {
+		return true
+	}
+	prefix := entry + "-"
+	nestedRoot := path.Dir(entry) + "/"
+	base := path.Base(entry)
+	for name := range files {
+		if strings.HasPrefix(name, prefix) {
+			return true
+		}
+		rest, ok := strings.CutPrefix(name, nestedRoot)
+		if !ok {
+			continue
+		}
+		slash := strings.IndexByte(rest, '/')
+		if slash <= 0 {
+			continue
+		}
+		leaf := rest[slash+1:]
+		if leaf == base || leaf == base+".exe" {
+			return true
+		}
+	}
+	return false
+}
+
+func uniqueStrings(values []string) []string {
+	seen := make(map[string]struct{}, len(values))
+	result := make([]string, 0, len(values))
+	for _, value := range values {
+		if _, exists := seen[value]; exists {
+			continue
+		}
+		seen[value] = struct{}{}
+		result = append(result, value)
+	}
+	return result
+}