feat: add independent AI air unit control system

[braedonsaunders]

Implement comprehensive air unit management so the AI controls air units as an independent tactical arm rather than lumping them with ground forces.

• AITacticsManager: separate air from ground army during attacks, air units flank perpendicular to main attack vector, support air (Lifter/Overseer) follows army centroid, air harassment against enemy economy, air defense
• FormationControl: air units positioned past enemy for flanking pincer, wide spacing (2x) to reduce splash vulnerability, independent priority
• AIMicroSystem: health-based disengage below 30%, hit-and-run perpendicular repositioning every 15 ticks, improved Valkyrie transform (1.5x threshold), retreat toward nearest friendly building
• AIScoutingManager: prefer idle flying units as scouts (bypass terrain)
• dominion.ts: counter-air on all difficulties, air superiority production (priority 75), increased Valkyrie/Specter priorities, specter in harass
• Tests: 6 new test sections covering air micro, army separation, flanking, transform decisions, support filtering, scout preference
• Docs: updated OVERVIEW.md and GAME_DESIGN.md with air control system

https://claude.ai/code/session_01JKVGQ8iVDUUp814udCRAq5