Fix tile outlines drawn over units (miniquad only depth-tests with depth writes); Soul Core starts with a treasury ring

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

DESIGN.md

@@ -56,7 +56,8 @@ An original dungeon management game (genre: dungeon building & defence) that run
 
 ## Game content
 - **Digging**: mark earth/gold → Pit Imps dig, bring gold home and claim ground
-- **Rooms**: treasury, lair, mushroom garden, training hall, library
+- **Rooms**: treasury, lair, mushroom garden, training hall, library (+ torture chamber, temple)
+- The Soul Core starts surrounded by a **one tile wide ring of treasury** (16 tiles)
 - Claim the **portal** → creatures: kobold, cave spider, shadow mage, stone ogre (depending on rooms)
 - **Needs**: sleep, food, wages (payday); unhappy creatures leave
 - **Hand**: pick up / drop own units (max. 8)