fix: AI units now engage nearby enemies instead of ignoring them in FFA

[braedonsaunders]

Previously, AI armies attacking a base would ignore other players' armies standing right next to them because units with active building targets never re-evaluated for higher-priority threats.

Four fixes working together for SC2-style combat behavior:

1. CombatSystem threat retarget: Units attacking buildings periodically check for enemy combat units within attack range and switch targets (every 15 ticks)
2. findThreatRetarget: New method searches attack range for units only (not buildings), using existing TargetAcquisition with includeBuildingsInSearch=false
3. AITacticsManager near-army threat detection: During attack operations, idle units are redirected toward nearby enemy combat units (2+ within 25 units) instead of being re-commanded to the committed enemy's buildings
4. Engagement buffer widened from attackRange+3 to attackRange+5 for more responsive target acquisition during attack-move marches

https://claude.ai/code/session_01DmLGJRBv2EMiF95WJXKABz