feat: session resumption and connection migration

[ayomidemariam]

Summary

Add encrypted session state persistence so clients can resume sessions across gateway restarts or reconnections without losing room membership.

What's included

• **** — class with AES-256-GCM encrypted storage, automatic key rotation, debounced saves, zlib compression, and both in-memory and Redis backends.
• **** — Integrated session resumption into the connection handshake: extracts from query, attempts restore on connect, debounces saves on room join/leave, and flushes on disconnect. Adds count to .
• **** — Flushes pending sessions and destroys the SessionManager on graceful shutdown.
• **** — 26 unit tests covering encryption, key rotation, delete, debounce, flushPending, in-memory mode, Redis mock, blob size limits, and destroy.
• **** — 10 integration tests covering full handshake scenarios, expired sessions, identity mismatch, debounce, room leave, metrics, and graceful shutdown.

Constraints respected

• No changes to auth.js, validator.js, rate-limiter.js, conn-rate-limiter.js, logger.js, errors.js, room-manager.js, geofence-engine.js, protocol-registry.js, distributed-room-manager.js, tls-manager.js, or admin-server.js
• No new npm dependencies — uses only Node.js built-in and
• All existing tests continue to pass (225 passed, 15 skipped)

Closes #258