feat: unreliable datagram support (QUIC + libp2p-datagram)

[royzah]

Description

Adds unreliable datagram support to libp2p, end to end:

• core: StreamMuxer::send_datagram / max_datagram_size and StreamMuxerEvent::Datagram (default: unsupported).
• quic: implements them over quinn datagrams, enabled by default and configurable.
• swarm: ConnectionEvent::Datagram (inbound) and ConnectionHandlerEvent::SendDatagram (outbound).
• New libp2p-datagram crate: a Behaviour + Control with send_datagram(peer, ...) and an IncomingDatagrams stream.

Motivation and prior art: libp2p/specs#626 (IP over libp2p), #225, and the MTU edge in #4647. Tunnelling IP over reliable, ordered streams is 2-10x slower (HOL blocking; TCP-over-TCP meltdown); a datagram carrier fixes it.

Notes

• Muxer trait methods default to unsupported, so other muxers are unaffected.
• quic::poll now surfaces connection close via the datagram future (was always Pending).
• Tests: quic datagram roundtrip, datagram-crate e2e over QUIC, swarm unit tests.

Open questions

• Datagrams default to on in quic::Config; happy to make it opt-in.
• Should the new enum variants be #[non_exhaustive]?

[jxs]

Hi, and thanks for starting this!
as commented on your specs post, wdyt of implementing libp2p/specs#680 and go from there? You're in need for this in production right? That would be a nice testbed for the spec

[royzah]

Nice, that's the direction we were hoping for. The quic/core/swarm bits here are the datagram primitive #680 needs anyway, so I'll keep those and rework the libp2p-datagram crate to match #680: /dg/1 control stream, app-proto-id prefix, Control StreamID varint per datagram. One extra bit I'll need is exposing the control stream's QUIC stream id through the muxer, since #680 keys on it. We'll run it on our radio mesh and report back.

[royzah]

Hey @jxs, while you're here, mind also casting an eye over #6487 (Control::open_stream_on_connection)? It's a small one and we need it in production too, alongside the datagram work.
WDYT ?

[royzah]

specs#680 is implemented here: /dg/1 control stream + app proto id, datagrams keyed by the control stream's QUIC id (via a new StreamMuxer::substream_id), inbound filtered by it. Tests green over QUIC. Radio-mesh bench next, will report numbers.

Two spec notes: one app-protocol per behaviour for now, and we drop on a bad/unknown stream id rather than raising PROTOCOL_VIOLATION (not reachable from this layer).