Convert all entity sprites to 4-frame animated spritesheets

[aebrer]

Feature Request

The barrel_fire 4-frame animation at 6 FPS looks incredible in-game — subtle flame flickering that brings the world to life. We should apply this treatment to ALL entity sprites.

Architecture (already in place)

The ENTITY_SPRITESHEETS system in entity_renderer.gd is generic and ready to go. To animate any entity:

1. Generate a 4-frame spritesheet (horizontal strip, e.g., 256x64 for 64x64 frames)
2. Add entry to ENTITY_SPRITESHEETS:

   "entity_type": {
       "path": "res://assets/textures/entities/entity_type_spritesheet.png",
       "frames": 4,
       "fps": 6.0,
   }

3. The system automatically creates AnimatedSprite3D with AtlasTexture regions

Entities to animate

Enemies (subtle idle animations — breathing, pulsing, shifting)

• bacteria_spawn — pulsing/throbbing organic mass
• bacteria_motherload — heaving, tentacles shifting
• bacteria_spreader — rotating spikes, chemical bubbling
• smiler — flickering grin, eyes shifting
• tutorial_mannequin — subtle sway or creepy stillness-then-twitch

Environmental

• barrel_fire — flame flickering (DONE, commit cbd6901)
• fluorescent_light — subtle warm glow pulse (very subtle, almost imperceptible)
• fluorescent_light_broken — occasional flicker/spark
• vending_machine — screen flicker, button lights
• exit_hole — swirling darkness, depth shimmer

Items (lower priority — currently use different renderer)

• Items use item_renderer.gd with Sprite3D, would need similar AnimatedSprite3D path

Notes

• Use the texture generation virtuous cycle (creator → critic → revise) for each spritesheet
• FPS should vary by entity: 6 FPS for fire, 2-3 FPS for subtle environmental, 4 FPS for enemies
• Keep the same 64x64 frame size for consistency
• Existing static textures stay as fallbacks in ENTITY_TEXTURES