Add Xbox split-screen support

[psalkowski]

Summary

Adds Xbox split-screen (couch co-op) support, allowing multiple players to join from a single Xbox console.

Background

This feature has been requested by the community (see Reddit, #750, #1459, #2521). Previous attempts were made in #976 and #3109 but were never merged.

Implementation

Sub-client handling

• Enabled SubClientLoginPacket processing (removed from ILLEGAL_SERIALIZER)
• Added setupSubClientSession() in LoginEncryptionUtils to authenticate sub-clients
• Sub-clients inherit locale and server address from parent session (not included in their JWT)

Disconnect detection

• Added DisconnectPacket handler to properly detect when sub-client leaves split-screen
• Sub-clients share the upstream connection with parent, so we don't close it on sub-client disconnect

Reconnect handling

• When sub-client rejoins after disconnect, CloudburstMC reuses the same BedrockServerSession
• We detect closed session and create a fresh GeyserSession for clean rejoin

Session tracking

• Added parentSession field to track sub-client relationships
• Added sessionByXuid() lookup in SessionManager

Testing

Tested on Xbox Series X with split-screen:

• Two players joining simultaneously
• Sub-client leaving and rejoining
• Parent disconnecting (sub-clients also disconnect)
• Server transfers between backend servers

Download

Binary file can be found here: https://github.com/psalkowski/Geyser/releases/download/v2.9.2-splitscreen/Geyser-Velocity-2.9.2-splitscreen.jar