refactor(quic): share the unified handshake authorization

Phase 11.5.4 — QUIC half. Findings M1, M9 on this transport.

quic_server._do_handshake_sync was a second, weaker copy of the WebRTC logic:
group_id was optional, so omitting it skipped the membership check entirely and
fell back to the node's first group (M1); node-scoped daemon tokens were
accepted as client tokens (M9); and the checks could drift from the WebRTC path
independently, which is how they diverged in the first place. Authorization now
comes from meshbay_common.handshake, shared with WebRTC.

C6 IS STILL OPEN ON THIS TRANSPORT. There is no GEK proof here yet: a forged or
stolen token still reaches the node over QUIC and can inject chat without
holding the group key. What remains is the challenge/response and the mutual
node proof — quic_binding() is written and unit-tested for exactly this, and
11.5.6 (whether a certificate hash is the right anchor, or an RFC 5705 exporter
is reachable from aioquic) is still unproven. This commit narrows the gap to
the proof itself; it does not close the finding.

QUIC tests updated: default tokens are members of the test group, and clients
pass group_id, since it is mandatory now.

Tests: 9 quic/multi-group, full node+common suite green.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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