Add client_transfer example

[mliem2k]

Summary

• No example or doc mention of the Transfer packet (added in 1.20.5) existed anywhere in this repo, despite the protocol data already fully supporting it.
• Adds examples/client_transfer, showing the correct way to follow a server-initiated transfer: end the current connection, wait for it to actually close, add a short grace period, then reconnect to the new host:port with the same username.
• That reconnect timing matters: an immediate reconnect can race the server's own cleanup of the old session and get rejected with "already playing on the server" (Bukkit/Paper's duplicate-login check). Hit this myself while building the example and fixed it with the wait-for-end + delay shown here.
• mineflayer has an open bug in the same area (Bug Report: Connection Error During Server Transfer mineflayer#3776: sends physics packets during the configuration phase a transfer triggers, which 1.21+ servers reject), so this seemed worth documenting at the protocol level directly.

Test plan

• npx standard examples/client_transfer/client_transfer.js passes with no changes needed
• Ran the example (using this repo's own src, not a published version) against a real Paper/Purpur 1.21.10 server performing a real transfer handoff between two hosts, confirmed both the initial login and the post-transfer login succeed:

  connecting to <host1>:25565
  logged in to <host1>:25565
  received transfer packet -> <host2>:25565, reconnecting
  connecting to <host2>:25565
  logged in to <host2>:25565
  

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