Skill cooldowns & 40 skills across 5 classes

Implemented the cooldown system for all combat skills. Each skill now has a turn-based cooldown that forces you to think about sequencing rather than spamming your strongest ability.

All 40 skills across the 5 classes are now fully implemented with rank progression and unique mechanics. Bulwark gets damage reduction walls. Signalborn channels alien frequencies. Flayer trades health for devastation.