feat: slime enemies — jump AI, contact damage, combat system

New entity: Slime (EntityKind::Slime)
- 19 parts: blobby body (3x5) + 2 glowing eyes
- Green translucent colors, brighter center
- HP: 25, spawns every 45 ticks (max 2 alive)

Slime AI (update_slime_ai):
- Jumps toward player every 60 ticks when within 40 cells
- Jump power scales with proximity (closer = stronger)
- Pauses horizontally between jumps (50/50 hop-stop cycle)
- Uses set_horizontal_vel + set_vertical_vel (vector movement)

Combat system (update_combat):
- AABB overlap check between player and all alive enemies
- Goblin: 8 damage per 20 ticks on contact
- Slime: 5 damage per 20 ticks on contact
- Knockback: player pushed away from enemy on hit
- Player.take_damage() called, death possible from enemies

World gen: try_spawn_slime() spawns at surface, 18 cells from player
Renderers: slime rendered as 's' in ascii/terminal, green blob in graphics

6 new tests (122 total, 0 failures)

src/render/vulkan.rs

@@ -217,6 +217,7 @@ impl VulkanRenderer {
                     let ch = match e.kind {
                         EntityKind::Player if e.alive => '@',
                         EntityKind::Goblin if e.alive => 'g',
+                        EntityKind::Slime if e.alive => 's',
                         _ => '%',
                     };
                     let fg = if e.on_fire {