feat: implement CookieBridge M1 — core protocol & relay server

- Protocol spec: encrypted envelope format, device identity (Ed25519 + X25519),
  LWW conflict resolution with Lamport clocks
- E2E encryption: XChaCha20-Poly1305 via sodium-native, X25519 key exchange
- WebSocket relay server: stateless message forwarding, device auth via
  challenge-response, offline message queuing, ping/pong keepalive
- Device pairing: time-limited pairing codes, key exchange broker via HTTP
- Sync protocol: envelope builder/opener, conflict-resolving cookie store
- 31 tests passing (crypto, pairing, conflict resolution, full integration)

Co-Authored-By: Paperclip <noreply@paperclip.ing>

src/index.ts

@@ -0,0 +1,39 @@
+export { RelayServer } from "./relay/index.js";
+export type { RelayServerConfig } from "./relay/index.js";
+
+export {
+  generateKeyPair,
+  deviceIdFromKeys,
+  serializeKeyPair,
+  deserializeKeyPair,
+  deriveSharedKey,
+  encrypt,
+  decrypt,
+  sign,
+  verify,
+  buildSignablePayload,
+} from "./crypto/index.js";
+export type { DeviceKeyPair, SerializedKeyPair } from "./crypto/index.js";
+
+export { PairingStore, generatePairingCode } from "./pairing/index.js";
+export type { PairingSession } from "./pairing/index.js";
+
+export { buildEnvelope, openEnvelope, CookieStore } from "./sync/index.js";
+
+export {
+  PROTOCOL_VERSION,
+  MESSAGE_TYPES,
+  MAX_OFFLINE_QUEUE,
+  PAIRING_CODE_LENGTH,
+  PAIRING_TTL_MS,
+} from "./protocol/spec.js";
+export type {
+  Envelope,
+  MessageType,
+  CookieEntry,
+  CookieSyncPayload,
+  PairingRequest,
+  PairingAccept,
+  PairingResult,
+  DeviceInfo,
+} from "./protocol/spec.js";