Add `air` and `fortification` armor classes to enable more nuanced
balance. Aircraft now use `air` class instead of light/medium, and
defensive turrets use `fortification` instead of `structure`, allowing
weapons to specialize against planes and defenses without affecting
other targets.
- Add armor classes to all 12 weapons with appropriate multipliers
- Adjust weapon stats (sniperrifle, rocketpod, shoulderrocket)
- Reduce HP on several combat units for balance
- Reclassify aircraft and defensive turrets to new armor classes
