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-sdk/src/backup.ts

@@ -132,15 +132,15 @@ export async function exportBackup(
 }
 
 /**
- * Import a backup blob, decrypt with the passphrase, and write all state
- * to the given storage (overwriting existing state).
+ * Decrypt a backup blob and return its payload without touching storage.
+ * Useful for peeking at the embedded identity (e.g. to fingerprint it for
+ * a verification gate) before any state is overwritten.
  */
-export async function importBackup(
+export async function decryptBackup(
   crypto: CryptoProvider,
-  storage: StorageProvider,
   blob: BackupBlob,
   passphrase: string,
-): Promise<void> {
+): Promise<BackupPayload> {
   if (blob.version !== BACKUP_VERSION) {
     throw new Error(`Unsupported backup version: ${blob.version}`);
   }
@@ -149,7 +149,6 @@ export async function importBackup(
   const nonce = fromBase64(blob.nonce);
   const ciphertext = fromBase64(blob.ciphertext);
 
-  // Derive the same key
   const key = await crypto.hkdf(
     new TextEncoder().encode(passphrase),
     salt,
@@ -164,38 +163,56 @@ export async function importBackup(
     crypto.zeroize(key);
     throw new Error('Wrong passphrase or corrupted backup');
   }
-
   crypto.zeroize(key);
 
-  const payload = JSON.parse(new TextDecoder().decode(plaintext)) as BackupPayload;
+  return JSON.parse(new TextDecoder().decode(plaintext)) as BackupPayload;
+}
 
-  // Restore identity
+/**
+ * Write a previously-decrypted backup payload to storage. Splitting this
+ * from `decryptBackup` lets callers run an interstitial verification gate
+ * (V3.3) before any pinned-trust entries are overwritten.
+ */
+export async function applyBackupPayload(
+  storage: StorageProvider,
+  payload: BackupPayload,
+): Promise<void> {
   if (payload.identity) {
     await storage.saveIdentityKeyPair(deserializeIdentityKeyPair(payload.identity));
   }
   await storage.saveLocalRegistrationId(payload.registrationId);
 
-  // Restore signed prekeys
   for (const spk of payload.signedPreKeys) {
     await storage.saveSignedPreKey(deserializeSignedPreKey(spk.data));
   }
 
-  // Restore one-time prekeys
   for (const otpk of payload.oneTimePreKeys) {
     await storage.saveOneTimePreKey(deserializeOneTimePreKey(otpk.data));
   }
 
-  // Restore sessions
   for (const s of payload.sessions) {
     await storage.saveSession(s.address, deserializeSessionState(s.state));
   }
 
-  // Restore trust
   for (const t of payload.trustedIdentities) {
     await storage.saveTrustedIdentity(t.address, fromBase64(t.key));
   }
 }
 
+/**
+ * Import a backup blob, decrypt with the passphrase, and write all state
+ * to the given storage (overwriting existing state).
+ */
+export async function importBackup(
+  crypto: CryptoProvider,
+  storage: StorageProvider,
+  blob: BackupBlob,
+  passphrase: string,
+): Promise<void> {
+  const payload = await decryptBackup(crypto, blob, passphrase);
+  await applyBackupPayload(storage, payload);
+}
+
 /** Serialize a backup blob to a compact single-string form (for copy/paste or QR). */
 export function backupToString(blob: BackupBlob): string {
   return `shade-backup:v${blob.version}:${blob.salt}:${blob.nonce}:${blob.ciphertext}`;