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