Port Bevy frontend to shared Sim core, wire in pixel art master
- bevy.rs is now a thin view over sim::Sim, mirroring the terminal frontend: continuous clock with bounded catch-up, heat-driven raids, pause/speed controls, unified keys (WASD/hjkl, m for schemes, SPACE pauses). Deletes ~700 lines of duplicated wave-loop game logic that had diverged from the terminal game. - Art resource loads the Pixel Lab assets with nearest-neighbor sampling: 12 textured tile types, tinted reuse for security doors and trap variants, flat-color fallback for the rest; villain, minions, henchmen, and agents render with their sprites. - Unpowered functional tiles render dimmed; villain gets a real sprite instead of the cursor square. - Hash derives on MinionClass/AgentType for texture maps. - README documents running the Bevy frontend via cargo run (asset root resolution); DESIGN.md marks Milestone 3 as started; devlog added.