Introduce a second detection layer independent of line-of-sight vision.
Radar buildings (Radar Tower, Advanced Radar) provide coverage that reveals
unit positions through fog as contacts, while Radar Jammers can blind enemy
radar within range without affecting normal sight.
Key changes:
- Add three sensor buildings: Radar Tower, Advanced Radar, Radar Jammer
- Implement computeRadar() with jamming logic and stampDisc() optimization
- Add isDetectedBy() for radar contact detection
- Draw radar contacts as green diamond blips in WorldView
- Show radar contacts as neutral green pips on minimap
- Expand build grid to 2 rows and artwork sheets to 4 rows
- Refuse repair orders on airborne units; builders stop chasing airborne targets
- Add validation for radarRange and jamRange in rules compiler
- Update sprites.md documentation and add comprehensive tests
