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/tests/ratchet.test.ts

@@ -31,6 +31,51 @@ async function setupPair(): Promise<{ alice: SessionState; bob: SessionState }>
 describe('Double Ratchet', () => {
   // ─── Basic Send/Receive ──────────────────────────────────
 
+  describe('initReceiverSession isolation', () => {
+    /**
+     * Regression — the V3.10 multi-sender recovery flow surfaced a
+     * bug where `initReceiverSession` shared a reference to the
+     * receiver's signed-prekey keypair with the new session. The
+     * first DH ratchet step zeroed the session's stale send-key
+     * private bytes — which were the SAME backing buffer as the
+     * persisted signed prekey. A subsequent X3DH from a different
+     * sender then derived a divergent root key and decryption
+     * failed.
+     *
+     * Fix: `initReceiverSession` copies the keypair into the
+     * session. Verify here.
+     */
+    test('does not mutate the caller-provided keypair after a DH ratchet step', async () => {
+      const rootKey = crypto.randomBytes(32);
+      const remoteIdentityKey = crypto.randomBytes(32);
+      const bobDH = await crypto.generateX25519KeyPair();
+      const originalPrivate = new Uint8Array(bobDH.privateKey);
+      const originalPublic = new Uint8Array(bobDH.publicKey);
+
+      const bob = initReceiverSession(rootKey, remoteIdentityKey, bobDH);
+
+      // Drive a full receive that triggers `performDHRatchetStep`
+      // via a message with a fresh dhPublicKey.
+      const aliceFirst = await initSenderSession(crypto, rootKey, remoteIdentityKey, bobDH.publicKey);
+      const msg = await ratchetEncrypt(crypto, aliceFirst, enc.encode('hi'));
+      await ratchetDecrypt(crypto, bob, msg);
+
+      // The ORIGINAL keypair must not have been touched, so a
+      // second X3DH-style establishment using the same prekey
+      // material still succeeds.
+      expect(Array.from(bobDH.privateKey)).toEqual(Array.from(originalPrivate));
+      expect(Array.from(bobDH.publicKey)).toEqual(Array.from(originalPublic));
+
+      // Sanity-check: a second receiver session built from the
+      // same keypair should still decrypt fresh sender traffic.
+      const bob2 = initReceiverSession(rootKey, remoteIdentityKey, bobDH);
+      const aliceSecond = await initSenderSession(crypto, rootKey, remoteIdentityKey, bobDH.publicKey);
+      const msg2 = await ratchetEncrypt(crypto, aliceSecond, enc.encode('hi again'));
+      const plain2 = await ratchetDecrypt(crypto, bob2, msg2);
+      expect(dec.decode(plain2)).toBe('hi again');
+    });
+  });
+
   describe('basic send/receive', () => {
     test('Alice encrypts, Bob decrypts', async () => {
       const { alice, bob } = await setupPair();