fix: balance with premade groups

[FrancescoBorzi]

Rework team selection so premade groups don't break balance

Team selection used to go through two separate code paths: a greedy newest-first loop when CFBG.EvenTeams.Enabled was off, and the pair-matching InviteSameCountGroups when it was on. Both had problems with premades:

• the greedy path could happily start a 3v1 because it never looked back at what it had picked
• the pair-matcher had a bug where rolled-back groups were erased from the selection pools
  without fixing the player count, leaving phantom players behind.

On top of that, the reinforcement path counted groups invited to other BG instances as if they were joining this one, and relied on GetFreeSlotsForTeam, which with Battleground.InvitationType = 0 imposes no relative cap at all.

So instead of patching each path separately, both match formation and the join-in-progress fill now go through a single routine (SelectBalancedGroups) that works in three steps:

1. stage groups onto the smaller team in queue order
2. then move/drop groups until the team sizes are within bounds (exact match with EvenTeams on, max diff of 1 otherwise)
3. then write the result into the selection pools once at the end.

Groups always stay whole, i.e. a premade never gets split across teams.

That made InviteSameCountGroups, InviteGroupToBG, CrossFactionQueueInfo and both GetLowerTeamIdInBG overloads dead code, hence removed.

No config changes; all existing options keep their meaning. The one deliberate behaviour change: above CFBG.EvenTeams.MaxPlayersThreshold, reinforcement now still keeps teams within 1 player instead of going fully uncapped.

Engineered using Claude Fable.

[PkllonG]

After testing, it doesn't affect normal use. I haven't noticed the author's situation, though.

[FrancescoBorzi]

thanks for your feedback @PkllonG !