Summary

• Fix inverted error condition (err != nil → err == nil) in listener/sing_tun/server.go:158
• The current code assigns tunName from getTunnelName only when the call fails, discarding the correct tunnel name on success
• This causes incorrect forwarderBindInterface behavior when FileDescriptor > 0 (Android/iOS scenarios)

Change

- if tunnelName, err := getTunnelName(int32(options.FileDescriptor)); err != nil {
+ if tunnelName, err := getTunnelName(int32(options.FileDescriptor)); err == nil {

Test plan

• Verify TUN mode works correctly on Android/iOS where FileDescriptor > 0
• Verify tunName is correctly set from the fd-based interface name
• Verify forwarderBindInterface is true only when getTunnelName succeeds

Fixes #2658

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