- New actors.rs: Rigidbody Actor with gravity, grid collision, powder displacement - Actor rendered as colored rectangle on pixel buffer (zoom-scaled) - WASM API: spawn_actor, move_actor, update_actor, actor_x/y - WASD/Arrow keys control actor, camera follows with soft lerp - displace_powders pushes sand/dirt out of actor area - Collision: stops at solids, walks through powders - Updated AGENTS.md with actor documentation