Migrate physics sim loop into Bevy ECS

[Copilot]

The physics simulation previously relied on a macroquad main loop. This change moves the simulation into Bevy’s ECS and rendering flow while keeping the same behaviors and controls.

• Simulation systems
  • Port collision, boundary resolution, and integration into Bevy Update systems
  • Centralize simulation configuration/state as resources
• Rendering + UI
  • Render balls via Mesh2d + ColorMaterial
  • Replace macroquad HUD with Text2d overlays anchored in centered coordinates
• Input + spawning
  • Handle mouse/keyboard input with Bevy input resources
  • Spawn balls at cursor with screen-to-world conversion

let world_cursor = Vec2::new(
    cursor.x - window.width() / 2.0,
    window.height() / 2.0 - cursor.y,
);

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