fix(engine): align DefaultConfig() zone parameters with plan §3.7.1

The DefaultConfig() function had incorrect zone parameters:
- ZoneStartTurn: 50 → 20 (2-player default per plan)
- ZoneShrinkInterval: 5 → 2 (shrink every 2 turns per plan)

This was causing the zone to start too late and shrink too slowly,
allowing bots to farm energy without being forced into combat.

The ConfigForPlayers() function already had correct values, but
DefaultConfig() is used in WASM builds (cmd/acb-wasm/main.go)
and various tests.

Closes: bf-4idw

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

engine/types.go

@@ -191,8 +191,8 @@ func DefaultConfig() Config {
 		EnergyInterval:     10,
 		CoresPerPlayer:     2,
 		ZoneEnabled:        true,
-		ZoneStartTurn:      50,
-		ZoneShrinkInterval: 5,
+		ZoneStartTurn:      20, // 2-player default per plan §3.7.1
+		ZoneShrinkInterval: 2,  // Shrink every 2 turns per plan §3.7.1
 		ZoneShrinkStep:     2,
 		ZoneMinRadius:      3,
 	}