message: Add Source Note about main loop performance
  The main loop's code is performance-critical, and this has implications
  on tasks design and planning. Knowing this earlier would help me avoid
  some major workflow inefficiencies while working on tasks touching this
  code.
diff: @@ -85,6 +85,8 @@ impl World {
  let main_loop_frame = main_loop.on_frame(
  f!([stats_monitor,on_before_frame,on_after_frame,uniforms,scene_dirty,scene]
  (t:animation::TimeInfo) {
  + // Note [Main Loop Performance]
  +
  stats_monitor.begin();
  on_before_frame.run_all(&t);
  @@ -113,6 +115,19 @@ impl World {
  .init()
  }
  + // Note [Main Loop Performance]
  + // ============================
  + // Any code repeated on each iteration of the Main Loop (each "frame") must be written with
  + // high care for performance. Any changes that has a chance of negatively impacting the
  + // constant overhead of the main loop needs *explicit* explanation, review, and acceptance *at
  + // design stage* of the proposed new implementation, from performance perspective, with an
  + // explicit note of the fact of Main Loop impact.
  + //
  + // Rationale: the "Main Loop" contains the code comprising a GUI rendering "frame" (term
  + // originating from a "still frame" term in filmmaking). The speed at which the Main Loop
  + // executes directly translates to the perceived performance of the GUI, and the FPS (frames
  + // per second) metric, impacting Users' experience with the application.
  +
  fn init(self) -> Self {
  self.init_composer();
  self.init_hotkeys();
  