Implement Figure-level overlay architecture with two-pass drawing - #32199

@ksunden @story645 Thanks for the feedback! I've updated the architecture based on your suggestion to make self.patch its own layer instead of special-casing it.

Here are the updates pushed in the latest commit:

1. Layer Dictionary: Replaced the hardcoded _overlay_children list and _overlay boolean flags with a more scalable _children_by_layer dictionary and a string layer argument.
2. Dedicated Patch Layer: self.patch is now fully integrated into the layer system inside its own "patch" layer.
3. Generic Draw Method: Replaced the hardcoded _draw_base_layer and _draw_overlay_layer methods with a single generic _draw_layer(renderer, layer) method. The draw() sequence for both Figure and SubFigure is now explicitly: "patch" -> "base" -> "overlay".
4. get_children: Updated get_children(layer=None) so it returns artists specific to the requested layer.
   When layer=None is passed, it safely returns the background patch followed by all the base and overlay artists

From the peanut gallery: We now have blend groups and modes #31162, and this is adding "layers" which is potentially similar to those. It would be useful to add some documentation on when a user would want to use one versus the other and also to make sure we are consistent in how we want to handle and thread these things through so they play nicely together and make sense from the defaults and argument names when appropriate. Maybe more critically my question is: Do we need a multi-pass drawing architecture here or should this actually be handled at the rendering level similar to what was done with the blend groups.