Support QUIC v1 alongside v2 for cross-language interop (Rust/quinn)

[espeed]

Problem

The Go bifrost QUIC configuration hardcodes Versions: []quic.Version{quic.Version2} in transport/common/quic/config.go:47. This means Go bifrost peers only accept QUIC v2 (RFC 9369) connections.

No pure Rust QUIC library implements QUIC v2:

• quinn (0.11.9) — no v2 support, open issue since 2023, not on roadmap
• s2n-quic (1.76.0) — no v2 support
• quiche (0.26.1) — no v2 support

This makes it impossible for a Rust bifrost peer to connect to a Go bifrost peer, even though every other protocol layer (protobuf wire formats, TLS certificate extensions, signaling, floodsub, SRPC) is wire-compatible.

What happens

A Rust peer (quinn) sends a QUIC v1 Initial packet. The Go peer (quic-go) rejects it because it only accepts v2. After adding v2 to quinn's supported_versions list, version negotiation passes but quinn can't parse v2 Initial packets (the long header packet types are permuted in v2).

Go error: timeout: no recent network activity
Quinn log: ignoring non-initial packet for unknown connection

Proposed fix

In transport/common/quic/config.go, change line 47 from:

Versions: []quic.Version{quic.Version2},

to:

Versions: []quic.Version{quic.Version2, quic.Version1},

This accepts both v1 and v2 connections. Go peers connecting to Go peers will still negotiate v2 (preferred). Rust peers will negotiate v1. The QUIC transport is otherwise identical — same TLS 1.3, same bifrost certificate extension (OID 1.3.6.1.4.1.53594.1.1), same ALPN ("bifrost"), same stream multiplexing.

Context

This comes from a Rust port of bifrost with 27 conformance tests proving byte-identical wire compatibility with Go for: peer IDs, SignedMsg, floodsub Packet, signaling SessionRequest/Response, WebRTC Signal, SRPC Packet, peer encryption, envelope, and KeyPEM format. The QUIC version is the only remaining interop barrier.

RFC 9369 (QUIC v2) is primarily an anti-ossification measure — the protocol behavior is identical to v1, only the packet header encoding and crypto salt differ. Accepting v1 alongside v2 has no security or functionality impact.