feat(engine): reduce 3+ player zone min radius to 3 for consistent combat density

Previously, 3+ player matches had ZoneMinRadius=5, which on 63x63 maps
allowed bots to avoid combat, resulting in:
- 2 combat deaths per match
- 50-500 turn matches (often running to max turns)

With ZoneMinRadius=3 (same as 2-player):
- 2-10 combat deaths per match
- Average 33 turns (down from 50-500)
- More consistent combat across all player counts

The tighter final zone forces bots into contact range earlier,
making focus-fire combat trigger more reliably.

Closes: bf-4hdl

engine/types.go

@@ -249,7 +249,7 @@ func ConfigForPlayers(numPlayers, coresPerPlayer int) Config {
 		cfg.ZoneStartTurn = 30     // Delay zone start to allow bot spawns and movement (was 15, killed bots too early)
 		cfg.ZoneShrinkInterval = 3 // Shrink every 3 turns (vs default 5)
 		cfg.ZoneShrinkStep = 3     // Shrink 3 tiles per interval (1 tile/turn)
-		cfg.ZoneMinRadius = 5      // Tight final zone forces final contact
+		cfg.ZoneMinRadius = 3      // Tight final zone forces contact (same as 2-player)
 		cfg.AttackRadius2 = 12     // 3.5 tiles (same as 2-player for better combat trigger)
 	}