Add automatic lobby-based Party Mode

[scottynou]

Summary

• automatically join Rose users from the same premade lobby in one relay room
• keep the room stable through champion select/game and reconnect/resync after interruptions
• keep custom mod files local; only selection metadata is shared

PARTY_MODE_ALWAYS_ON controls the automatic flow; the manual token path remains available when it is disabled.

Test

python -m unittest tests.test_party_reliability -v — 16 passed