feat(engine): reduce zone min radius to force combat contact

For 3+ player matches, zone now shrinks to radius 2 (down from 3) to force
bots into attack range (~2.24 tiles). Combined with earlier zone start (turn 20
vs 50) and faster shrink (3 vs 2 tiles per interval), this creates a forcing
function for combat contact.

Closes: bf-5htl

engine/types.go

@@ -235,6 +235,14 @@ func ConfigForPlayers(numPlayers, coresPerPlayer int) Config {
 	// Scale energy nodes with player count
 	cfg.EnergyInterval = 10
 
+	// Scale zone parameters for 3+ players to force combat contact
+	// 2-player matches naturally converge at center; 3+ need aggressive zone
+	if numPlayers >= 3 {
+		cfg.ZoneStartTurn = 20  // Start shrinking earlier
+		cfg.ZoneShrinkStep = 3  // Shrink faster (3 tiles per interval vs 2)
+		cfg.ZoneMinRadius = 2   // Force bots into tighter cluster (attack radius is ~2.24)
+	}
+
 	return cfg
 }