Strategy bots (GathererBot, RusherBot, SwarmBot) now move toward zone
center when within 2 tiles of the zone edge, not just when outside.
This anticipates the shrinking zone and prevents bots from moving
away from center due to energy-seeking logic.
Test results: 60% of matches now have combat deaths (3/5), up from 0%.
Zone margin of 2 tiles aligns with engine's built-in bot behavior.
Fixes gap identified in test replay where gatherer bot at distance 5
from center (zone radius 6) moved away from center and died from
zone_death instead of engaging in combat.
Closes: bf-2ham
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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