[andig]

On macOS with many multicast interfaces (here 22: anpi*, en*, awdl0, llw0, utun*), a burst of per-interface multicast writes on one shared UDP socket can wedge that socket. sendmsg first fails with ENETUNREACH, then blocks forever. A fresh socket per interface and repeated writes on a single interface do not trigger it.

Server.Shutdown calls unregister, which sends the goodbye multicast synchronously, so Shutdown never returns. Every caller hangs with it. In ship-go, go test ./mdns/ runs into the 10 minute test timeout, both on dev (zeroconf v2) and on enbility/ship-go#68 (v3):

goroutine 1304 [IO wait, 1 minutes]:
internal/poll.(*FD).RawWrite
...
github.com/enbility/zeroconf/v2.(*Server).multicastResponse  server.go:793
github.com/enbility/zeroconf/v2.(*Server).unregister          server.go:659
github.com/enbility/zeroconf/v2.(*Server).Shutdown            server.go:244
github.com/enbility/ship-go/mdns.(*ZeroconfProvider).UnannounceService

Fix

• ipv4PacketConn.WriteTo and ipv6PacketConn.WriteTo set a 100 ms write deadline per packet. A healthy send takes microseconds.
• isInterfaceGone treats os.ErrDeadlineExceeded as gone. InterfaceManager.MarkFailed then backs the interface off, so the next send does not stall on it again.

Verification (macOS, behind a corporate VPN)

• zeroconf go test .: before, TestBasic hit the timeout after 6m40s. After, the suite completes in 28 s. TestBasic, TestSubtype and TestQuickShutdown still fail on this machine because multicast does not work there. They are not hangs, and TestQuickShutdown fails on dev too.
• ship-go#68 with this branch via replace: go test ./mdns/ before, Test_GoroutineLeakOnShutdown hung into the timeout. After, it completes in 76 s. Only the discovery assertions of Test_ZeroConf fail, for the same multicast reason.
• New tests: TestWriteToSetsDeadline (fails without the fix) and TestIsInterfaceGone_WriteDeadlineExceeded_ReturnsTrue.

The existing TestIsInterfaceGone_ETIMEDOUT_ReturnsFalse still passes. A kernel ETIMEDOUT stays transient. Only our own write deadline counts as gone.

ship-go still pins zeroconf v2, so this reaches ship-go once enbility/ship-go#68 lands and picks up this commit.

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