feat: implement industry-standard fog of war optimizations

[braedonsaunders]

Implements all three phases of fog of war improvements based on research from StarCraft 2, League of Legends, Civilization VI, and other AAA RTS games.

Phase 1: Performance Foundation (Reference Counting)

• VisionOptimizer: Track how many units can see each cell
• Cell boundary detection: Only update when units cross grid boundaries
• Incremental updates: O(moved_units × affected_cells) vs O(casters × cells)

Phase 2: Gameplay Critical (LOS Blocking)

• LineOfSight: Height-based terrain vision blocking
• Ray-tracing and shadowcasting algorithms
• Integration with Terrain heightmap data
• Units on high ground see down, low ground can't see up

Phase 3: Visual Polish (SDF Edges)

• SDFVisionRenderer: Signed distance field for smooth fog edges
• Pattern-based upscaling (LoL's 16 transition patterns)
• Resolution-independent edge quality without expensive blur

Key Changes

• src/engine/systems/vision/VisionOptimizer.ts: Reference counting system
• src/engine/systems/vision/LineOfSight.ts: Terrain LOS blocking
• src/engine/systems/vision/SDFVisionRenderer.ts: SDF edge rendering
• src/engine/systems/VisionSystem.ts: Integration of all optimizations
• src/rendering/compute/VisionCompute.ts: Height texture support for GPU
• src/rendering/Terrain.ts: Expose heightmap for vision calculations
• Comprehensive test coverage for all new systems

https://claude.ai/code/session_01XSmJ7hgNws3pkW6EHC2W4j