[FR] Per-team unit limit manipulation

I would like to be able to manipulate the per-team unit limit. For my purposes I don't need to change the total unit limit. I just need to be able to transfer the unit limit between teams.

The common use-case will be that if one team leaves then their unit limit should be transferred to the player who receives their units. The team may return (they were not killed) so their unit limit must be able to transfer back to the original team.

Ideally this would work when called from inside `gadget:AllowUnitTransfer` such that a transfer can always be guaranteed to succeed.

Start by tracing the gadget:AllowUnitTransfer entry point and the existing total unit-limit handling. Determine where per-team limits are stored and transferred, including how a transfer can be reversed when a team returns. Done means the per-team limit can move between teams without changing the total limit and can be used reliably from gadget:AllowUnitTransfer.

Written by the indexing model from the issue text.