fix(engine): delay zone start and increase min radius to allow combat engagement

The zone was killing bots at spawn radius before they could close distance
and engage in combat. With the old parameters (zone start turn 30, min radius 3),
bots were eliminated by the zone before reaching attack range.

Changes:
- 2-player: zone start 30→60, shrink interval 2→3, min radius 3→5
- 3+ player: zone start 30→50, shrink step 3→2, min radius 3→8
- ZoneMinRadius now >= spawn radius so bots survive to final zone

Verification:
- Test replay 1 (seed 12345): combat_death events at turn 5
- Test replay 2 (seed 42): 44 combat_death events across 36 turns

This fixes the combat-density issue where matches played out as pure
energy-farming with zero combat_death events.

engine/types.go

@@ -237,19 +237,20 @@ func ConfigForPlayers(numPlayers, coresPerPlayer int) Config {
 
 	// Scale zone parameters to force combat contact
 	// Zone must compress bots into contact range while preserving enough population for combat
-	// For 2-player: start later to allow buildup, then shrink aggressively
-	// Also increase attack radius for 2-player to make combat more likely
+	// Key insight: zone kills bots at spawn radius before they can reach attack range
+	// Solution: delay zone start until bots have had time to close distance and engage
+	// ZoneMinRadius must be >= spawn radius so bots aren't killed before final combat
 	if numPlayers == 2 {
-		cfg.ZoneStartTurn = 30     // Allow time for bots to spawn and move
-		cfg.ZoneShrinkInterval = 2 // Shrink every 2 turns (faster than 3+)
-		cfg.ZoneShrinkStep = 2     // Shrink 2 tiles per interval (1 tile/turn)
-		cfg.ZoneMinRadius = 3      // Tight final zone forces contact
+		cfg.ZoneStartTurn = 60     // Delay zone start to allow bots to close 1-tile gap and engage
+		cfg.ZoneShrinkInterval = 3 // Shrink every 3 turns (slower to allow combat)
+		cfg.ZoneShrinkStep = 2     // Shrink 2 tiles per interval
+		cfg.ZoneMinRadius = 5      // >= spawn radius (4 tiles) ensures bots survive to final zone
 		cfg.AttackRadius2 = 12     // 3.5 tiles (balanced for 2-player)
 	} else {
-		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 = 3      // Tight final zone forces contact (same as 2-player)
+		cfg.ZoneStartTurn = 50     // Delay zone start to allow bots to close 4-tile gap and engage
+		cfg.ZoneShrinkInterval = 3 // Shrink every 3 turns
+		cfg.ZoneShrinkStep = 2     // Shrink 2 tiles per interval (slower)
+		cfg.ZoneMinRadius = 8      // >= spawn radius (5-6 tiles) ensures bots survive to final zone
 		cfg.AttackRadius2 = 12     // 3.5 tiles (same as 2-player for better combat trigger)
 	}