[Refactor] Use imgui-java for FlixelDebugOverlay debugging UI

[stringdotjar]

FlixelDebugOverlay is currently built with libGDX primitives (SpriteBatch, ShapeRenderer, BitmapFont, etc.) for FPS/stats, a log console, watch panel, and optional hitbox drawing. That works, but it is limited compared to a full immediate-mode GUI.

Requirements:

• Integrate imgui-java (Dear ImGui bindings for Java) for the debug overlay so we get:

  • Resizable/dockable windows, trees, tables, and better scrolling
  • Easier layout and more room for future tools (profiling hooks, entity pickers, toggles, filters)
  • A familiar pattern for game/engine debug UIs