`/ally` works unilaterally

Anybody in an allyteam[^1] can do `/ally` and the engine just applies given alliance ([ceasefire](https://github.com/beyond-all-reason/RecoilEngine/issues/1656)) state. There is no way for other teammates to disagree (other than just unilaterally applying a different state), and no way for a game to say anything about it.

It would be good if this was up to game control.

Perhaps all there is to do would be to provide a basecontent gadget that listens to `/luarules ally` and calls `Spring.SetAlly` under a faithful replication the existing engine behaviour, then get rid of native `/ally`. `fixedAllies` would be a modoption.

[^1]: I didn't see any spectator check, I hope this doesn't mean that spectators can also control allyteam 0's ceasefire state or that you can desync a replay this way. Something to check and/or handle properly in the replacement gadget.

Start by tracing the native `/ally` entry point and the `/luarules ally` path, then inspect how a basecontent gadget could call `Spring.SetAlly`. Check spectator access and replay/desync behavior, and compare against the existing ceasefire behavior. Done means game control is possible without native `/ally`, with `fixedAllies` exposed as a modoption and the existing behavior faithfully preserved.

Written by the indexing model from the issue text.