v0.2.1 — Exit the host app on close code 4000

When the server closes the connection with code 4000 (the device token was just claimed by a newer connection), the SDK now exits the host app (exit(0)) after logging to stderr/logcat, instead of staying stopped.

• One token pair belongs to one device seat — reconnecting after a takeover would ping-pong with the servers kicker forever.
• Fail-loud by design: a token accidentally shipped in a release build can no longer keep a silent debug channel alive. Gate start() on kDebugMode.
• The stderr line right before the exit means the shutdown leaves a visible trace in logcat (Android) / Console (iOS) — previously the log only went to the in-memory ring buffer, which dies with the process.
• A user-initiated stop() never exits.

Install: omnidebuglink: {git: {url: https://github.com/omnidebuglink/omnidebuglink_flutter.git, ref: v0.2.1}}