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-key-transparency/src/util.ts

@@ -0,0 +1,42 @@
+/**
+ * Constant-time byte comparison. Mirrors `@shade/core` `constantTimeEqual`
+ * but is duplicated here so the KT package has no runtime dependency on
+ * `@shade/core` for primitive comparisons (it depends on it for error
+ * types only).
+ */
+export function constantTimeEqual(a: Uint8Array, b: Uint8Array): boolean {
+  if (a.length !== b.length) return false;
+  let diff = 0;
+  for (let i = 0; i < a.length; i++) {
+    diff |= a[i]! ^ b[i]!;
+  }
+  return diff === 0;
+}
+
+/**
+ * Encode bytes to standard base64 (no URL variant). Avoids platform-specific
+ * `Buffer` so the KT package works in browsers, Bun, and Workers.
+ */
+export function toBase64(bytes: Uint8Array): string {
+  if (typeof Buffer !== 'undefined') return Buffer.from(bytes).toString('base64');
+  let str = '';
+  for (let i = 0; i < bytes.length; i++) str += String.fromCharCode(bytes[i]!);
+  return btoa(str);
+}
+
+export function fromBase64(s: string): Uint8Array {
+  if (typeof Buffer !== 'undefined') return new Uint8Array(Buffer.from(s, 'base64'));
+  const str = atob(s);
+  const out = new Uint8Array(str.length);
+  for (let i = 0; i < str.length; i++) out[i] = str.charCodeAt(i);
+  return out;
+}
+
+/** Stable hex encoding for log_id rendering. */
+export function toHex(bytes: Uint8Array): string {
+  let s = '';
+  for (let i = 0; i < bytes.length; i++) {
+    s += bytes[i]!.toString(16).padStart(2, '0');
+  }
+  return s;
+}