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_test.go

@@ -0,0 +1,29 @@
+package protocol
+
+import "testing"
+
+func TestPaymentRPCBackendCandidatesPreferPlatformTaggedFiles(t *testing.T) {
+	darwin := PaymentRPCBackendCandidates("backend/provider", "darwin", "arm64")
+	if got, want := darwin[0], "backend/provider-darwin-arm64"; got != want {
+		t.Fatalf("darwin candidates[0] = %q, want %q (%v)", got, want, darwin)
+	}
+	windows := PaymentRPCBackendCandidates("backend/provider", "windows", "amd64")
+	if got, want := windows[0], "backend/provider-windows-amd64.exe"; got != want {
+		t.Fatalf("windows candidates[0] = %q, want %q (%v)", got, want, windows)
+	}
+	if windows[len(windows)-1] != "backend/provider" {
+		t.Fatalf("windows fallback = %v", windows)
+	}
+}
+
+func TestHasAnyPaymentRPCBackendAcceptsTaggedArtifacts(t *testing.T) {
+	if HasAnyPaymentRPCBackend("backend/provider", map[string][]byte{"backend/provider-darwin-arm64": {1}}) != true {
+		t.Fatal("tagged darwin provider should be accepted")
+	}
+	if HasAnyPaymentRPCBackend("backend/provider", map[string][]byte{"backend/windows-amd64/provider.exe": {1}}) != true {
+		t.Fatal("nested windows provider should be accepted")
+	}
+	if HasAnyPaymentRPCBackend("backend/provider", map[string][]byte{"backend/unrelated": {1}}) {
+		t.Fatal("unrelated backend file should not satisfy the payment entry")
+	}
+}