moq-native,relay: don't advertise the illegal qmux-00.moqt-18 pair

[kixelated]

Summary

moq-transport-18 requires qmux-01, so qmux-00.moqt-18 is an illegal WebSocket subprotocol pair. On dev the Rust side advertised the full {qmux-01, qmux-00} × moq_net::ALPNS matrix (relay) and offered every draft per ALPN (&[], native client/listener), which produces that illegal pair. js/net's connect.ts already pins draft-18 to qmux-01, so the Rust side was the outlier; this brings it in line.

• rs/moq-native/src/websocket.rs: add a private qmux_versions_for(alpn) helper that pins moqt-18 to qmux-01 and returns &[] (every draft) for all other ALPNs. Used by both the WebSocket client connect() and the WebSocketListener.
• rs/moq-relay/src/websocket.rs: supported_subprotocols() skips the qmux-00.moqt-18 cell of the matrix.
• Each crate guards the "moqt-18" literal with a test asserting it stays the IETF draft-18 ALPN (wire 0xff000012), so the pin can't silently rot.

No new public API. js/net already encodes this policy, so no JS change is needed.

Background

This supersedes #1513, which added a moq-net version→qmux policy mapping (QmuxVersion, qmux_versions(), QMUX_ALPNS, QMUX_ALPN_STRINGS, etc.) as a workaround for qmux 0.1.0. dev's qmux 0.1.1 with_protocols(alpn, &[Version]) API plus the full-matrix supported_subprotocols() already handle qmux negotiation, so that surface is redundant. The only real gap was the one illegal pair, which this fixes minimally.

Test plan

• cargo test -p moq-native -p moq-relay --lib websocket:: — passes, including:
  • moqt_18_pins_to_qmux01 / supported_subprotocols_lists_full_matrix: the pin and the advertised matrix (with the illegal pair excluded), plus draft-18 literal drift guards.
  • every_advertised_pair_is_acceptable: spins up the axum WebSocket handler and, for every versioned (qmux, moq) pair we advertise, connects a client offering exactly that pair and asserts it negotiates end-to-end; also asserts offering qmux-00.moqt-18 fails the handshake.
  • axum_ws_negotiates_newest_moq_alpn: existing newest-ALPN regression, refactored onto a shared spawn_test_server harness.
• cargo fmt -p moq-native -p moq-relay -- --check clean.
• cargo clippy -p moq-native -p moq-relay --all-targets -- -D warnings clean.

(Written by Claude)