gadget:AllowWeaponTarget is uncessarily overloaded

If targetID is -1, then it means "please tell me the target acquisition range for this slow update". This extra use was added in 2018, and as far as I know, raaar (Metal Faction) is still using it: https://springrts.com/mantis/view.php?id=6096

Overloading the callin like this is confusing, so I split the new meaning out into AllowUnitTargetRange. See:
- https://github.com/ZeroK-RTS/Zero-K/commit/1a38d8380ef6134c337838c00ba6d4c60a62f3b7
- https://github.com/ZeroK-RTS/Zero-K/blob/master/LuaRules/gadgets.lua#L1343-L1377

Perhaps the split could happen engine-side.

Start by reading the AllowWeaponTarget usage described in the issue and the linked Zero-K LuaRules/gadgets.lua section at lines 1343-1377, then compare the referenced commit. Determine the engine-side callin boundaries and confirm that target acquisition range no longer requires an overloaded targetID value. Done means the two meanings are exposed as separate callins without breaking the existing consumer.

Written by the indexing model from the issue text.