PR summary

This PR introduces the foundational OverlayManager to backend_bases.py, setting up the architecture required for high-frequency interactive overlays (like cursors and crosshairs) without triggering expensive background redraws.

Key Changes:

• Added OverlayManager to manage registered overlay artists using weak references.
• Attached OverlayManager to FigureCanvasBase initialization.
• Implemented Phase 1 (Fallback Mode): Standard canvases automatically fall back to setting animated=False on artists and routing redraws to draw_idle().
• Added unit tests in test_backend_bases.py to verify the fallback behavior, registration logic, and a future-looking test (currently marked XFAIL) for native compositing in Phase 2.

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 checklist

• "closes #0000" is in the body of the PR description
• new and changed code is tested
• Plotting related features are demonstrated in an example
• New Features and API Changes are noted with a directive
• Documentation complies with guidelines