feat(engine): increase 3+ player attack radius for consistent combat_density

Problem: Some 4-player seeds had 0 combat_death events (e.g., seed 100).
Root cause: AttackRadius2=9 (3 tiles) was too small for the ~10 tile
spawn separation on 63x63 maps.

Solution: Increase AttackRadius2 to 12 (3.5 tiles) for 3+ players,
matching the 2-player configuration.

Results:
- 4-player: 5/5 seeds now have combat_death (was 4/5)
- Seed 100: 4 combat_death (was 0)
- 2-player: no regression (still 2 combat_death per match)
- 3-player: 2-3 combat_death per match (all have combat)

Closes: bf-omj2

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

engine/types.go

@@ -250,6 +250,7 @@ func ConfigForPlayers(numPlayers, coresPerPlayer int) Config {
 		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.AttackRadius2 = 12     // 3.5 tiles (same as 2-player for better combat trigger)
 	}
 
 	return cfg