feat(protocol): MNP 4.0 flag day for the node-audience token (B2)

The node-audience token (previous commit) is a change to what a peer must
present, so it is a MAJOR per the versioning rule (§5.6): a pre-4.0 client
presents its hub session token and a 4.0 node refuses it, and there is no
compatibility branch, because leaving one would keep a hub credential reachable
by every node (C6's lesson). So the floor moves with the version.

- MNP_VERSION 3.4 -> 4.0 and MNP_MIN_SUPPORTED 3.0 -> 4.0 (meshbay_common);
  transport.js MNP_V/MNP_V_MIN -> 4.0 to match.
- MIN_CLIENT_VERSION 0.13.0 -> 0.16.0 so a stale desktop client is told to
  update before connecting rather than meeting a handshake refusal it cannot
  read; the browser reloads this build from the hub.
- Regenerate tests/golden/dispatch.json: the only change is the `v` the node
  stamps on outbound messages, 3.4 -> 4.0 (56 cases, v field only).
- Document the split and the flag day: MESHBAY_DESIGN.md §5.2 (the handshake
  token is the MNP-audience token), §5.6 (the 4.0 flag day), register E10 and
  decision 23; MESHBAY_NODE_PROTOCOL.md §6.3 (authorize_token binds MNP_AUD)
  and the wire-version banner.

Deploy is coordinated and atomic (common+hub+node+SPA together); a live
browser-to-node validation and the deploy itself remain. common (173), node
(1489, the pre-existing test_cli_golden argparse/prog artifact aside) and hub
(1471) suites all green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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