netutil: preserve UDP peer address after packet release

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Background

SockaddrSliceToUDPAddr currently returns a net.IP slice that aliases the raw sockaddr memory in the UDP receive buffer.

After Packet.Free() or ReadFrom returns, that buffer may be reused by a later receive operation. The peer IP retained by the caller can then be overwritten while the port remains unchanged, causing responses to be lost or sent to the wrong peer.

Changes

• Copy IPv4 and IPv6 address bytes into storage owned by the returned net.UDPAddr.
• Add regression tests that overwrite the original sockaddr buffer and verify that the decoded address remains unchanged.

Compatibility

• No exported API changes.
• Each conversion adds owned backing storage for 4 bytes on IPv4 or 16 bytes on IPv6.
• Port and IPv6 zone handling remain unchanged.

Validation

• go test -race -count=10 ./internal/netutil
• go test ./...
• go vet ./...

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[Rememorio]

LGTM