cgo_bridge: export CGoSetEnv so hosts can reach xray-core env knobs

cgo_bridge/main.go

@@ -6,6 +6,7 @@ package main
 import "C"
 
 import (
+	"os"
 	"unsafe"
 
 	libXray "github.com/xtls/libxray"
@@ -23,3 +24,22 @@ func CGoInvoke(requestJSON *C.char) *C.char {
 func CGoFree(value *C.char) {
 	C.free(unsafe.Pointer(value))
 }
+
+// CGoSetEnv sets an environment variable inside the Go runtime.
+//
+// The Go runtime snapshots environ when the c-archive image initializes, so a
+// libc setenv() performed later by the host application is never visible to
+// os.LookupEnv. That makes xray-core's env-based knobs unreachable from a
+// non-Go host — most importantly platform.TunFdKey ("xray.tun.fd"), which
+// proxy/tun/tun_darwin.go reads to adopt an existing tun descriptor.
+//
+// On iOS the descriptor only exists after NEPacketTunnelProvider.startTunnel,
+// long after the snapshot, so the "iOS: use provided fd from NetworkExtension"
+// branch can never be taken: NewTun falls through to creating its own utun via
+// an AF_SYSTEM socket, which the app sandbox denies, and the inbound fails with
+// "app/proxyman/inbound: failed to start proxy > operation not permitted".
+//
+//export CGoSetEnv
+func CGoSetEnv(name *C.char, value *C.char) {
+	_ = os.Setenv(C.GoString(name), C.GoString(value))
+}