A unit can only have 1 working stockpile weapon

* one of the stockpile weapons will be assigned as "the" stockpile weapon on a unit.
* Lua interfaces apply "to the unit", i.e. to that weapon, and you can't specify a different one.
* native stockpile command will only apply to ammo for that weapon. This is probably fine if you can use Lua, but the command could have a weapon index parameter.
* the other weapons are still technically stockpile weapons so can't fire due to being stuck at 0 ammo.

The issue names Lua interfaces and the native stockpile command; start by tracing those entry points and how a unit selects its stockpile weapon. No files or tests are identified. Done should mean that each stockpile weapon can be addressed independently and that additional stockpile weapons are not left unable to fire.

Written by the indexing model from the issue text.