feat(engine): set ZoneMinRadius=12 for 50% combat_death rate

After testing 300+ matches with various ZoneMinRadius values:
- ZoneMinRadius=3: 0% combat (kills bots too fast)
- ZoneMinRadius=8: 24% combat
- ZoneMinRadius=12: 50% combat (best so far)
- ZoneMinRadius=15: 38% combat

ZoneMinRadius=12 provides the best balance between forcing proximity
and preserving bot population for combat encounters.

Target: 90% combat_death rate (still need +40 points)

Related: bf-4pm8 (Combat Density epic)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

engine/types.go

@@ -242,7 +242,7 @@ func ConfigForPlayers(numPlayers, coresPerPlayer int) Config {
 		cfg.ZoneStartTurn = 20     // Start shrinking early (vs default 50)
 		cfg.ZoneShrinkInterval = 3 // Shrink every 3 turns (vs default 5)
 		cfg.ZoneShrinkStep = 3     // Shrink 3 tiles per interval (1 tile/turn)
-		cfg.ZoneMinRadius = 15     // Preserve more population for combat
+		cfg.ZoneMinRadius = 12     // Balance proximity and population (50% combat_death observed)
 	}
 
 	return cfg