release(v4.0.0): Shade GA — V3.x consolidation + audit prep

V3.1 → V3.12 consolidated and tagged for the first GA release. Wire
format unchanged from 0.4.x — 4.0 peers interoperate with 0.4.x peers
byte-for-byte. The version bump is semantic: audit-cycle complete,
opt-in surface fully exposed, threat model refreshed for every new
surface.

Highlights:
- All 24 @shade/* packages bumped to 4.0.0 in lockstep.
- CHANGELOG 4.0.0 section is the canonical manifest of what landed.
- THREAT-MODEL extended (§10 fingerprint gates, §11 WebRTC P2P, §12
  Web-Worker boundary) + residual-risks table refreshed.
- OpenAPI now covers all 27 routes: prekey, transfer, KT, inbox,
  bridge, observer, /metrics, /healthz, /ready.
- MIGRATION 0.3.x → 4.0 documented + smoke-tested against
  shade migrate-storage on a real SQLite DB.
- docs/audit/REVIEW-BUNDLE.md + SCOPE.md ready for external reviewer.
- scripts/soak.ts harness for the GA-stable 2-week soak window.
- All V*.md plans archived under docs/archive/ with Status: Done.
- Voice/Video carved out into V5.0; 4.0 audit focuses on the frozen
  non-realtime stack.

Tests: TS 1000/1000 + Kotlin 11/11 cross-platform vectors green.
Docker: gt.zyon.no/stian/shade-prekey:4.0.0 builds and reports
  version 4.0.0 on /health.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/shade-core/src/ratchet.ts

@@ -71,7 +71,11 @@ export async function initSenderSession(
  * Initialize a session as the receiver (Bob, after X3DH).
  *
  * Bob knows the root key and his own signed prekey (which was used as
- * the initial DH ratchet keypair).
+ * the initial DH ratchet keypair). The keypair is COPIED into the
+ * session — the receiving side's DH ratchet will eventually rotate
+ * `dhSend` and zeroize the previous private key, and that scratch
+ * buffer must NOT be the same memory as the persisted signed prekey
+ * (which is shared with future X3DH establishments from other senders).
  */
 export function initReceiverSession(
   rootKey: Uint8Array,
@@ -83,7 +87,10 @@ export function initReceiverSession(
     rootKey,
     sendChain: { chainKey: new Uint8Array(32), counter: 0 },
     receiveChain: null,
-    dhSend: localDHKeyPair,
+    dhSend: {
+      publicKey: new Uint8Array(localDHKeyPair.publicKey),
+      privateKey: new Uint8Array(localDHKeyPair.privateKey),
+    },
     dhReceive: null,
     previousSendCounter: 0,
     skippedKeys: new Map(),