The enemy spawning code was starting to become a mess,

so I rebuilt it as a wave system.There are now 12 planned waves.

Each one controls which enemies appear, how many spawn, and how quickly they enter. Once a wave is cleared, the game pauses for a moment, displays a warning message, and then begins the next one.

This made a much bigger difference than I expected. The game now has a proper sense of progression instead of feeling like enemies are appearing randomly forever.