tnet: support graceful restart

[Rememorio]

Summary

• Add optional Restartable for services that support graceful restart while keeping Service unchanged.
• Add TCP graceful restart that starts a child process with the listener fd, stops accepting new connections, and waits for active connections to drain.
• Add UDP graceful restart that starts a child process with the first packet connection fd and closes the old packet connections.
• Add WithGracefulRestartTimeout and targeted restart tests.

Compatibility

This keeps the existing Service interface source-compatible. Callers that need restart can opt in with a type assertion to Restartable; existing custom Service implementations do not need to add a new method.

Validation

• go test . -run 'Restart|TCPConnWritevOutboundBufferLimitExceeded|TCPService|UDPService|ServiceClose|ConvertPacketConn' -count=1
• go test . -count=1
• go test $(go list ./... | grep -v '/internal/reuseport') -count=1
• go vet $(go list ./... | grep -v '/internal/reuseport')

Note: local go test ./... -count=1 still fails in internal/reuseport on this macOS environment; the PR changes do not touch that package. Local golangci-lint is v2 and cannot load this repository's v1 config, so lint is left to CI.

[WineChord]

LGTM