Naval Warfare: Multi-ship battles stall

[Noordfrees]

Describe the bug
Sea battles involving more than 2 ships at a time can stall indefinitely.

To reproduce
Steps to reproduce the behavior:

1. Play a LAN game where you control two players with warships. To maximize the chances of observing the bug, we'll say that player 1 has a warship with many soldiers, and player 2 has two warships with no soldiers.
2. Send all three ships into close proximity of each other.
3. Player 2 attacks p1's ship with both ships at once (one attack with 2 attackers selected in the attack window).
4. If p1's ship is sunk by the first attacker, everything is fine. But if p1-ship survives the first battle, then the battle between p2's second ship and p1-ship hangs. Save-reloading does not fix it, and the savegame fails to load in debug builds because the battle stacks of both ships are totally messed up.

Expected behavior
After p2's first ship sinks, p2's other ship and p1's ship start fighting.

Related
Ships actively fighting cannot be attacked. But actually, attacking a fighting ship should be allowed and behave the same as attacking one ship with multiple attackers. In all cases, the attackers need to queue for their turn to engage with the selected enemy. It seems like they all engage all at once currently, which damages the battle stacks.

Version:

• OS: Debian Unstable
• Widelands Version: 15dc1ea (Tournament2023)