Add manual text staggering for overlapping labels

[cvanelteren]

This adds an explicit text-staggering helper for dense figures without changing the default behavior of text placement. Axes.stagger_text() can now offset existing text() and annotate() artists horizontally, vertically, or in two dimensions to reduce overlap, and Figure.stagger_text() applies the same behavior across selected subplots while keeping each subplot independent. The PR also adds targeted regressions for text, annotation, idempotency, and figure-level selection behavior. Closes #617.

[Copilot]

Pull request overview

Adds an explicit “stagger text” utility to UltraPlot axes (and a figure-level convenience wrapper) to automatically offset overlapping Text/Annotation artists, with tests validating overlap reduction and idempotency.

Changes:

• Implement Axes.stagger_text(...) to iteratively offset text/annotation artists in display space until their bboxes no longer overlap.
• Implement Figure.stagger_text(...) to apply axes-level staggering across a figure’s selected subplots.
• Add unit tests covering vertical/horizontal/both-direction staggering, figure-level defaults, and idempotency.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
ultraplot/axes/base.py	Adds the core staggering algorithm and helpers on Axes.
ultraplot/figure.py	Adds axes selection helper and a figure-level stagger_text wrapper.
ultraplot/tests/test_axes.py	Adds tests for axes-level staggering across directions and idempotency.
ultraplot/tests/test_figure.py	Adds tests for figure-level staggering across subplots and selection.

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