feat!: MNP 1.0 — seal index and handshake_ack under the group key

`index_sync`, `index_delta` and the `handshake_ack` config payload now travel
sealed under a GEK-derived subkey (`meshbay_common/groupbox.py`, mirrored by
`sealGroup`/`openGroup` in `crypto.js`). Only `type`, `v`, `group_id` and the
ack's `node_pk`/`proof`/`sig` stay in clear — a receiver must route and
authenticate before it would trust a decryption. Verify, then decrypt.

The ack line is integrity, not confidentiality: the signed handshake transcript
names no ack field, so `is_node_admin`, `enabled_apps`, `video_root` and the
rest were authenticated by the DTLS channel alone. The index line is defence in
depth against a repeat of C1/C6 — a peer served before the handshake completes
now gets ciphertext, not filenames. Nothing against an observer, the hub, or a
member; that is the whole claim. `index_progress` stays clear (D3, counters
only). Chat is out of scope.

Failure is fatal: a payload that does not open ends the session naming the
message type — never an empty index or an empty `enabled_apps`, both of which
are legitimate states.

Version negotiation ships here too (phase 15.6, brought forward): `v` + `v_min`
on `handshake` and `handshake_challenge`, refused with `version_too_old` /
`version_too_new` / `version_unreadable`. The flag day was already being paid
for; the next breaking change now costs a refusal message.

BREAKING CHANGE: breaks the WebRTC wire every deployed client speaks. Hub and
every node must deploy together; the SPA is served by the hub, so a browser
picks up the new client on reload. See MESHBAY_NODE_PROTOCOL.md §11.1a, §13.1.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HkzbhmMmK8PqQBtGz5zCvY

1 files changed, 1 insertions, 1 deletions

diff --git a/docs/devel-phases-next.md b/docs/devel-phases-next.md
index 48620ca..e842b62 100644
--- a/docs/devel-phases-next.md
+++ b/docs/devel-phases-next.md
@@ -1109,7 +1109,7 @@ flags. Additions that belong in the user-facing docs:
 | 15.3 | Node: decrypt chat on receive | Decrypt incoming chat messages, handle out-of-order |
 | 15.4 | Key rotation on removal | Member removed **or device revoked or unpinned** → all remaining devices rotate |
 | 15.5 | Chat retention config | Per-group `max_age_days` setting, periodic cleanup in ChatStore |
-| 15.6 | MNP version negotiation | Handshake declares a supported version range, not a single `v` field (L2 — today `v` is sent by everyone and checked by no one). **Move this earlier**: it pairs with the minimum-client-version check, and version skew begins the day the desktop client ships (`desktop-client-v1.md` §2.6), not when chat is encrypted |
+| 15.6 | MNP version negotiation | ✅ **DONE 2026-09-03**, and not here: it shipped with **MNP 1.0** (the sealed index and ack), which forced a coordinated deployment anyway. `handshake` and `handshake_challenge` each carry `v` and `v_min`; `check_version` refuses with `version_too_old` / `version_too_new` / `version_unreadable`, shaped like `not_a_member`. The flag day was already being paid for, so the next breaking change costs a refusal message instead of a second one. See `MESHBAY_NODE_PROTOCOL.md` §13.1 |
 | 15.7 | Chat attachments | Attachments are ordinary files on the node and remain plaintext at rest. Either encrypt them under the sender key, or document the asymmetry explicitly. Note they now land in the **operator-designated upload root** (§6.7 of the desktop-client doc) |
 
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