ENH: Add subfigure_mosaic for creating SubFigure layouts

[nathanhu0]

Summary

• Implements subfigure_mosaic method that creates SubFigure layouts using the same ASCII art or nested list syntax as subplot_mosaic
• Refactored existing subplot_mosaic to share implementation via a private _do_mosaic method
• Addresses [ENH]: subfigure_mosaic matplotlib/matplotlib#25949

Implementation Details

• Extracted common mosaic logic into _do_mosaic with dependency injection for the child creation method
• subplot_mosaic passes self.add_subplot as the child creator
• subfigure_mosaic passes self.add_subfigure as the child creator
• All subfigures created are flat (direct children of the calling figure/subfigure)
• Users can create hierarchies by calling subfigure_mosaic on a SubFigure

Testing

• Added 5 comprehensive tests in TestSubfigureMosaic class
• Tests cover: basic functionality, string input, empty cells, width/height ratios, and nested hierarchies
• All 40 existing TestSubplotMosaic tests still pass without modification

API Decisions

• sharex/sharey parameters not supported for subfigures (they're containers without axes at creation time)
• Users can still share axes between subfigures after creating axes within them
• Follows same flat structure approach as subplot_mosaic - nesting in spec is for layout only

Test plan

• Run new subfigure_mosaic tests: pytest lib/matplotlib/tests/test_figure.py::TestSubfigureMosaic
• Verify existing tests still pass: pytest lib/matplotlib/tests/test_figure.py::TestSubplotMosaic
• Add gallery example (future work)
• Add What's New entry (future work)

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