Add UltraLayout: Advanced constraint-based positioning for non-orthogonal subplot arrangements

[cvanelteren]

Closes #297,#270

This PR introduces UltraLayout, a novel constraint-based layout system using kiwisolver that enables aesthetically pleasing positioning for non-orthogonal subplot arrangements (e.g., [[1, 1, 2, 2], [0, 3, 3, 0]] where subplot 3 should be centered between subplots 1 and 2). The existing gridspec works great for orthogonal layouts but fails to produce visually balanced results when subplots span multiple rows/columns in non-aligned ways. UltraLayout automatically detects non-orthogonal layouts, applies constraint satisfaction to compute optimal positions that respect spacing, ratios, and aesthetic balance (e.g., centering subplots between neighbors), and gracefully falls back to the standard grid layout if kiwisolver is not installed or for orthogonal arrangements. This enhancement is fully backward compatible, adds zero overhead for standard grid layouts, includes comprehensive test coverage in ultraplot/tests/test_ultralayout.py, and provides a foundation for more sophisticated layout constraints in the future.

Make subplot param probes backend-safe: GridSpec.get_subplot_params now returns a read-only snapshot of our margins/spaces and locally_modified_subplot_params returns False. This keeps Positron/other backends from erroring while preserving our restriction on mutating Matplotlib’s subplot params.

[cvanelteren]

Made a local error with the history so this is a restored version of the old PR.

[codecov]

Codecov Report

❌ Patch coverage is 70.45658% with 330 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
ultraplot/axes/base.py	45.90%	218 Missing and 20 partials ⚠️
ultraplot/gridspec.py	65.66%	36 Missing and 21 partials ⚠️
ultraplot/ultralayout.py	86.71%	21 Missing and 13 partials ⚠️
ultraplot/figure.py	50.00%	0 Missing and 1 partial ⚠️

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