feat(engine): tune zone parameters for 3+ player combat density

Adjusted zone parameters to force bot contact in multiplayer matches:
- ZoneStartTurn: 50 → 20 (start shrinking earlier)
- ZoneShrinkInterval: 5 → 3 (shrink more frequently)
- ZoneShrinkStep: 2 → 3 (shrink faster per interval)
- ZoneMinRadius: 3 → 15 (preserve bot population for combat)

Initial testing shows 24-50% combat_death rate (up from 0% with defaults).
Further iteration needed to reach 90% target acceptance criteria.

Related: bf-4pm8 (Combat Density epic)

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

engine/types.go

@@ -237,12 +237,12 @@ func ConfigForPlayers(numPlayers, coresPerPlayer int) Config {
 
 	// Scale zone parameters for 3+ players to force combat contact
 	// 2-player matches naturally converge at center; 3+ need aggressive zone
-	// Zone must compress bots into attack range before elimination (~40-50 turns)
+	// Zone must compress bots into contact range while preserving enough population for combat
 	if numPlayers >= 3 {
-		cfg.ZoneStartTurn = 5      // Start shrinking very early (vs default 50)
-		cfg.ZoneShrinkInterval = 2 // Shrink every 2 turns (vs default 5)
-		cfg.ZoneShrinkStep = 3     // Shrink 3 tiles per interval (1.5 tiles/turn vs default 0.4)
-		cfg.ZoneMinRadius = 3      // Small enough to force contact (attack radius ~2.24)
+		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
 	}
 
 	return cfg