feat(engine): reduce zone min radius to force combat contact

Reduce ZoneMinRadius from 10 to 3 to force bots into a smaller
safe area (6-tile diameter vs 20-tile diameter). With attack
radius of ~2.24 tiles, bots in the final zone will be within
combat range and trigger focus-fire combat_death events.

Before: 20-tile diameter safe zone (area = 314 tiles)
After: 6-tile diameter safe zone (area = 28 tiles)

This is a pure config parameter change - no combat algorithm
or scoring changes.

Closes: bf-30jt

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

engine/types.go

@@ -194,7 +194,7 @@ func DefaultConfig() Config {
 		ZoneStartTurn:      50,
 		ZoneShrinkInterval: 5,
 		ZoneShrinkStep:     2,
-		ZoneMinRadius:      10,
+		ZoneMinRadius:      3,
 	}
 }