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

PR summary

This PR introduces a foundational Figure-level overlay architecture to figure.py. It implements a two-pass drawing system, allowing developers to cleanly segregate base plot artists from overlay artists.

Key Changes:

• Segregated Artist Storage: Introduced _overlay_children to FigureBase. Figure elements are now routed to either _children (base layer) or _overlay_children (overlay layer).
• Two-Pass Drawing: Overhauled Figure.draw() to execute in two distinct passes:
  • _draw_base_layer(): Renders the figure patch (background) and all artists in _children.
  • _draw_overlay_layer(): Renders all artists in _overlay_children.
• Public API Routing: Added an _overlay=False keyword-only argument to key artist insertion methods (add_artist, text, legend, figimage).
• Title/Label Support: Plumbed the _overlay keyword through the suptitle, supxlabel, and supylabel family (via _suplabels) down to the underlying text calls.

Addresses #30515

AI Disclosure

AI tools were used to assist in drafting text and suggesting validation scenarios.
All code changes, final implementation decisions, and verification were done manually.

PR quality check

• Use an expressive title, e.g. "Fix title font property precedence"
• New and changed code is tested
• Plotting related features are demonstrated in an example
• New features and API changes have release notes
• Documentation complies with general and docstring guidelines