improvements, part 4

another update already? i've been busy!

this one's a pretty big update, with many changes to the base engine:

✱ enemies can now use cards!
✱ enemies now have unique attack values
✱ 4 new cards!
✱ new enemy: Yggdra
✱ loot drops
✱ unique spawns: no longer spawns the same enemy twice
✱ added some stats to the top of the screen
✱ extended the music loop
✱ various bug fixes & adjustments