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, 32 insertions, 2 deletions

diff --git a/docs/MESHBAY_DESIGN.md b/docs/MESHBAY_DESIGN.md
index c6d3cc7..394dfb5 100644
--- a/docs/MESHBAY_DESIGN.md
+++ b/docs/MESHBAY_DESIGN.md
@@ -16,7 +16,7 @@
 > them — it names the invariant that holds today, not the incident that produced
 > it. §13 is the register of those labels.
 >
-> Wire versions at the time of writing: **MNP 3.4** (oldest peer accepted 3.0),
+> Wire versions at the time of writing: **MNP 4.0** (oldest peer accepted 4.0),
 > **MHP 0.1**, packages **0.16.0**. The normative source for the wire format is
 > `MESHBAY_NODE_PROTOCOL.md`; this document states the design the protocol
 > serves, not its byte layout.
@@ -992,12 +992,29 @@ already fails the key proof. Pinning covers the case where an attacker *holds* t
 group key — an ex-member, a leaked key — and swaps the node underneath, which the
 proof alone cannot distinguish from the genuine node.
 
+**The token the member presents is a node-audience token, never the hub session
+token (E10).** A member hands whatever it presents here to the node operator,
+who is in the threat model, so the credential must open nothing at the hub. The
+hub signs two audiences with its one key: a session token (`aud` = the hub API)
+for `hubFetch` and signaling, and a short-lived **MNP token** (`aud = MNP_AUD`,
+from `POST /v1/nodes/mnp-token`) that carries the member's `sub`, `groups` and
+`jti` and is the only thing presented in the handshake. The node binds `MNP_AUD`
+when it decodes, so a session token is refused here; the hub API binds its own
+audience, so an MNP token captured by an operator is refused there. It is
+checked once, before the proof, so its short life never interrupts a transfer or
+a film already playing — a reconnect fetches a fresh one. `meshbay_common/tokens.py`
+holds the two audience strings, shared by the hub that issues and the node that
+verifies.
+
 **Authorization rules:**
 
 - `group_id` is **mandatory**. Omitting it once skipped the membership check and
   fell back to the node's first group (**M1**).
 - `scope == "user"` is enforced by default; node-scoped daemon tokens are refused
   on the client path (**M9**, **NS7**).
+- **The audience must be `MNP_AUD`.** A hub session token, or one with no
+  audience, is refused — a member's credential to a node is not a credential to
+  the hub (**E10**).
 - The denylist is consulted for user, `jti` **and** group.
 - The node **refuses connections when it holds no group key** — there is no
   `gek_required: false` bypass (**NS8**).
@@ -1229,6 +1246,16 @@ client on reload; the desktop client ships its own UI, which is why
 connecting and says "this version can no longer connect" rather than showing a
 handshake refusal nobody can act on.
 
+**MNP 4.0 (2026-09-25) is the worked example of a MAJOR that had to be one.** A
+member now presents a node-audience token in the handshake, not its hub session
+token (§5.2, **E10**), so a pre-4.0 client — which presents the session token —
+cannot authenticate to a 4.0 node, and a 4.0 client cannot authenticate to a
+pre-4.0 node, which expected the session token. Leaving a branch that still
+accepted the session token would keep the very disclosure the change closes
+reachable on every node, which is exactly what "no compatibility switch" forbids.
+So the floor moved to 4.0, `client.minimum` moved to the release that carries the
+new client, and the hub, node and SPA deploy together.
+
 **Every *requirement* is true of every peer the client can reach.** The floor moves
 with each MAJOR, so `check_version` refuses at the handshake any peer that cannot
 meet one: an upload is sealed or it is not sent; a transfer has a real lease or it
@@ -3210,6 +3237,7 @@ had already been asked.
 | **E7** | **Several named roots** per group, forming one virtual root (§6.2) |
 | **E8** | **exFAT/NTFS and Windows are the common case.** Linux ships first; that is build order, not population (§10) |
 | **E9** | **Group-related server state lives on the node. Always** (§1.3) |
+| **E10** | **The credential a member presents to a node is not the one the hub API accepts.** A member hands its handshake token to the node operator, who is in the threat model, so it is a short-lived node-audience token (`aud = MNP_AUD`, §5.2), useless at the hub API; the session token (`aud` = hub API) is never disclosed to a node. Before this the member presented its hub session token, so an operator held a live hub credential for the member — enough to enumerate their other groups and act as them at the hub. Closed by the audience split and the MNP 4.0 flag day (§5.6) |
 | **O1** | Initial key setup in the pre-proof window — deferred; that window is where C4 and C5b came from |
 | **O2** | A LAN enrolment door — one endpoint, bounded window, one-time code, closing permanently on success |
 | **O3** | `device_policy {allow_bundle: false}`, signed by a pinned key — **the mechanism that actually closes C4** (§3.7) |
@@ -3251,6 +3279,7 @@ had already been asked.
 20. **Private keys never leave the device on native clients.** Qualified: a browser has no durable storage of its own and still needs a bundle on each node, so C4 closes for an *account* only when it opts out of browser use.
 21. **Hub minimisation is enforced by an acceptance test, not by policy.** The hub must be *unable* to see keys, content or file listings.
 22. **No new code exchanges between people.** Safety numbers are refused for identity verification, permanently. The device-linking code is between a person's own devices and is unaffected. The total user-visible cost of the whole authorship story is **one notice**: *"this account's key changed"*.
+23. **A member's credential to a node is audience-separated from its hub credential** (**E10**, §5.2). The handshake carries a short-lived `aud = MNP_AUD` token, never the hub session token; the hub API accepts only its own audience. A node operator therefore holds nothing that acts at the hub.
 
 ### 14.2 Client architecture
 
diff --git a/docs/MESHBAY_NODE_PROTOCOL.md b/docs/MESHBAY_NODE_PROTOCOL.md
index 3351793..b3cd6eb 100644
--- a/docs/MESHBAY_NODE_PROTOCOL.md
+++ b/docs/MESHBAY_NODE_PROTOCOL.md
@@ -1,6 +1,6 @@
 # MeshBay Node Protocol (MNP)
 
-**Wire version:** `3.4` — `meshbay_common/__init__.py` (`MNP_VERSION`)
+**Wire version:** `4.0` — `meshbay_common/__init__.py` (`MNP_VERSION`)
 **Oldest peer accepted:** `3.0` — `handshake.py` (`MNP_MIN_SUPPORTED`)
 **Normative implementation:** `meshbay-common` (`protocol.py`, `handshake.py`,
 `groupbox.py`, `chatbox.py`, `adminop.py`, `join.py`, `device.py`, `crypto.py`,
@@ -462,6 +462,7 @@ absent. `verify_proof` compares with `hmac.compare_digest`.
 | Rule | Refusal | Rationale |
 |---|---|---|
 | JWT verifies under the hub's Ed25519 public key (`EdDSA`) | `Invalid JWT: ...` | |
+| `aud == MNP_AUD`, and `exp`/`sub`/`scope` present (MNP 4.0) | `Invalid JWT: ...` | the member presents a short-lived **node-audience** token (`POST /v1/nodes/mnp-token`), not its hub session token — the operator holds whatever is presented, and the session token opens the hub API. The two audience strings are in `meshbay_common/tokens.py` |
 | `scope == "user"` | `Wrong token scope` | a node-scoped daemon token must not be usable as a client token |
 | `sub` non-empty | `Token has no subject` | |
 | `group_id` non-empty | `group_id is required` | an absent group means no membership check to make; there is no default group, and a node's first group is not one |