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-recovery/src/encoding.ts

@@ -0,0 +1,45 @@
+/**
+ * Encoding helpers shared by the setup, request, and guardian modules.
+ * Kept as a tiny standalone module so individual flows don't carry
+ * private base64 helpers; consistent encoding across send/receive
+ * sides.
+ */
+
+/** Base64url (no padding) — used for both `recoveryKey → passphrase` and arbitrary share bytes. */
+export function bytesToBase64Url(bytes: Uint8Array): string {
+  let bin = '';
+  for (let i = 0; i < bytes.length; i++) bin += String.fromCharCode(bytes[i]!);
+  return btoa(bin).replace(/\+/g, '-').replace(/\//g, '_').replace(/=+$/, '');
+}
+
+export function base64UrlToBytes(s: string): Uint8Array {
+  const padded = s.replace(/-/g, '+').replace(/_/g, '/');
+  const padding = padded.length % 4 === 0 ? 0 : 4 - (padded.length % 4);
+  const bin = atob(padded + '='.repeat(padding));
+  const out = new Uint8Array(bin.length);
+  for (let i = 0; i < bin.length; i++) out[i] = bin.charCodeAt(i);
+  return out;
+}
+
+/**
+ * Convert a `recoveryKey` (32 random bytes) to the passphrase that
+ * `Shade.exportBackup` / `Shade.importBackup` expect. We use base64url
+ * because:
+ *  - it's a string, satisfying the export/import API,
+ *  - 32 bytes encodes to 43 characters, comfortably above the 12-char
+ *    minimum the exportBackup helper enforces,
+ *  - the encoding is deterministic so split + reconstruct + decode
+ *    yields the identical passphrase the original device used.
+ *
+ * The HKDF inside `exportBackup` is a deterministic KDF that's
+ * cryptographically appropriate for a 32-byte uniformly-random IKM
+ * (this is exactly the standard HKDF use case). The fact that the
+ * passphrase API was designed for human-typed passwords does not
+ * weaken the construction here.
+ */
+export function recoveryKeyToBackupPassphrase(key: Uint8Array): string {
+  if (key.length !== 32) {
+    throw new Error(`recoveryKey must be 32 bytes (got ${key.length})`);
+  }
+  return `shade-rk:${bytesToBase64Url(key)}`;
+}