The previous ZoneMinRadius=5 created a final zone diameter of 10 tiles,
which allowed bots to remain outside the 3.5-tile attack radius even when
both were inside the zone. This resulted in low combat_death rates for
passive bot strategies (~10% for random bots vs the 65-80% target).
With ZoneMinRadius=3, the final zone diameter is 6 tiles, forcing bots
into proximity where focus-fire combat triggers more consistently.
Also adds verify-combat-density.sh script for ongoing metrics tracking.
Closes: bf-4bj9
