fix(engine): set DefaultConfig ZoneMinRadius to 1 per plan §3.7.1

DefaultConfig() is used as the base for ConfigForPlayers(), which
overrides ZoneMinRadius based on player count (2 for 2-player, 1 for
3+). The default should match the most common case (3+ players).

Per plan §3.7.1: ZoneMinRadius=1 for 3+ players, 2 for 2-player.

Closes: bf-6985

engine/types.go

@@ -194,7 +194,7 @@ func DefaultConfig() Config {
 		ZoneStartTurn:      10, // Per plan §3.7.1 (both 2-player and 3+)
 		ZoneShrinkInterval: 1,  // Per plan §3.7.1 (both 2-player and 3+)
 		ZoneShrinkStep:     2,  // Per plan §3.7.1 (both 2-player and 3+)
-		ZoneMinRadius:      3,
+		ZoneMinRadius:      1,  // Per plan §3.7.1: 3+ player default (ConfigForPlayers overrides for 2-player)
 	}
 }