feat: phase 3 CLI + phase 4 JS client library

CLI (cmd/axon):
- Add explicit Unsubscribe on req exit (after EOSE and on Ctrl-C)
- Add reconnect with exponential backoff (1s→30s) for req --stream

JS library (js/axon.js):
- Canonical tag encoding and payload construction matching Go byte-for-byte
- Ed25519 sign/verify, keypair generation, challenge signing
- AxonClient: WebSocket connect with auth handshake, publish, subscribe,
  unsubscribe, reconnect-ready callback API
- encryptDM/decryptDM: X25519 ECDH (Ed25519 key conversion) + ChaCha20-Poly1305
- runVectors: validates all 6 Phase 1 test vectors against Go ground truth

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