forwarder: recreate stale connected UDP sockets

Summary

Connected UDP sockets can remain bound to a source IP that disappears after a host network change. Writes then keep failing with EADDRNOTAVAIL, leaving long-lived UDP flows broken indefinitely.

Retire the stale socket when a write returns EADDRNOTAVAIL, redial, and retry that datagram once. Track socket identity so the old reply loop cannot remove a replacement connection when it exits.

Fixes #648.

Testing

• go test ./pkg/services/forwarder -count=1
• go test -race ./pkg/services/forwarder -run TestUDPProxy -count=1
• git diff --check

The full go test ./... run did not complete: it entered the QEMU end-to-end suite and was stopped while its setup was still running.