Standalone UDP socket capability with one outstanding receive credit.

The consumer receives {:quic_udp, generation, credit, remote, bytes, at} and calls consumed/3 after processing it. Until then the socket stays passive. Excess traffic remains in the bounded OS receive buffer or is dropped; it is not forwarded into an unbounded consumer mailbox.

Binds must be concrete IP addresses. Ancillary destination metadata and multihoming are not supported. Send completion means local UDP acceptance, never peer acknowledgement. Times use monotonic microseconds.