test(relay): cross-version qmux WebSocket backward-compat

[kixelated]

Stacked on #1810 (needs the qmux 0.2.0 bump from that PR). Merge #1810 first, then rebase this onto main.

Why

The qmux 0.0.8 → 0.2.0 upgrade in #1810 touches the only transport with clients already deployed in the wild: WebSocket. These tests prove old clients still interop, and lock that in so a future qmux bump can't silently break them.

QUIC/WebTransport is unaffected (not qmux), and tcp:///unix:// are brand-new with no old peers — so WebSocket is the whole compatibility surface.

What's added

• old_qmux_client_interops_over_websocket — the real test. Adds qmux 0.0.8 as a renamed dev-dependency (qmux_old) and connects an actual old client to a server running the current qmux with the relay's real supported_subprotocols(). Asserts:

  • they converge on the newest lite ALPN carried over the legacy qmux-00 wire (the old client only knows qmux-00, the server prefers qmux-01), and
  • a uni stream round-trips b"hello" — exercising qmux-00 frame encode (old) → decode (current) across the version gap.

  Both qmux versions depend on web-transport-trait = "0.3", so a single unified trait drives both sides — no two-trait plumbing.

• advertises_legacy_subprotocols_for_old_clients — asserts the relay keeps advertising the bare webtransport / qmux-00 fallbacks and qmux-00.<newest-lite>, the subprotocols old browser/native clients depend on. A regression guard so a refactor (or version bump) can't drop them unnoticed.

Notes

• This is the in-tree half of the "make sure we don't break old clients" plan. The other half (manual: old published @moq/watch against a branch-built relay; canary deploy) is operational and not in this PR.
• Deeper qmux-00 framing edge cases are qmux's own test responsibility; this verifies the moq-relevant guarantee (negotiation + a stream round-trip end-to-end).

Test plan

• cargo test -p moq-relay --lib websocket — both new tests pass alongside the existing negotiation test
• cargo fmt --check + clippy --all-targets -D warnings

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(Written by Claude)