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>

2 files changed, 14 insertions, 2 deletions

diff --git a/packages/meshbay-common/src/meshbay_common/__init__.py b/packages/meshbay-common/src/meshbay_common/__init__.py
index 216a18f..842c52d 100644
--- a/packages/meshbay-common/src/meshbay_common/__init__.py
+++ b/packages/meshbay-common/src/meshbay_common/__init__.py
@@ -241,5 +241,13 @@ __version__ = "0.16.0"
 # `invite_link_result` and `invite_cancel`, a code bound to no account until it
 # is redeemed. Additive in the same way — a 3.3 node answers `unknown message
 # type`, and no client can hold a link code for a node that could not issue one.
-MNP_VERSION = "3.4"
+#
+# 4.0 (2026-09-25) is a MAJOR — a change to what a peer must *present*, not an
+# additive message (§5.6). A member now authenticates to a node with a
+# short-lived MNP-audience token (aud=MNP_AUD), never its hub session token: the
+# node operator holds whatever is presented, and the session token opens the hub
+# API. A pre-4.0 client presents the session token and is refused at the
+# handshake — there is no compatibility branch, because leaving one would keep
+# the disclosure reachable on every node. So the floor moves with it.
+MNP_VERSION = "4.0"
 MHP_VERSION = "0.1"
diff --git a/packages/meshbay-common/src/meshbay_common/handshake.py b/packages/meshbay-common/src/meshbay_common/handshake.py
index 2393df1..dedfeb1 100644
--- a/packages/meshbay-common/src/meshbay_common/handshake.py
+++ b/packages/meshbay-common/src/meshbay_common/handshake.py
@@ -85,7 +85,11 @@ CHALLENGE_PREFIX = b"meshbay:mnp:challenge:v1"
 # has no vocabulary to understand — or serving it outside every cap the operator
 # set, which makes the caps decoration. Neither is honest, so it is refused
 # here, with a code and a sentence.
-MNP_MIN_SUPPORTED = "3.0"
+# 4.0 (2026-09-25): a member authenticates with an MNP-audience token, not the
+# hub session token (MNP_VERSION note). A pre-4.0 peer presents the session
+# token, which this node now refuses — so the floor moves to 4.0 rather than
+# leaving a branch that would keep a hub credential reachable by every node.
+MNP_MIN_SUPPORTED = "4.0"
 
 ROLE_CLIENT = "client"
 ROLE_NODE = "node"