[max-models]

A plot can now be saved as a web figure, the format of maxplotlib.web (max-models/maxplotlib#85). The file holds Plotly JSON without plotly.js and without a look of its own. The docs theme's PlotlyFigure (max-models/starlight-theme#53) embeds it in the site's light and dark colours.

• result.save("x.plotly.json", alt=..., title=..., aspect=..., white_plot=...) writes a web figure. A Matplotlib result is converted to Plotly first, and animations keep their frames and slider. A plain .json is unchanged and still writes the figure as it is. save_figure's default plotly.json now writes the web figure.
• result.to_json(web=True, **hints) returns the same text. Without web=True it is unchanged, and hints raise a TypeError.
• plotly_backend.web_figure(fig, **hints) wraps maxplotlib.web.web_figure. Without maxplotlib it raises an ImportError that names the new web extra (pip install "plasma-plots[web]").
• Notebook output: with maxplotlib installed, the application/vnd.plotly.v1+json of display_bundle carries the palette slots and the title hint of a web figure. A docs site built from the notebook then recolours it like a saved .plotly.json. The output keeps its own template, size and backgrounds for Jupyter, and without maxplotlib it is the plain JSON as before.
• The Plotly guide describes the new format, and API.md is regenerated.

The extra asks for maxplotlibx>=0.2, but maxplotlib.web is only on maxplotlib devel, so the floor needs bumping to the release that ships it before plasma-plots is released. On main the new tests skip until then.

Checks: 7 new tests in tests/test_web_figures.py. Locally 1027 tests pass and 15 are skipped. One vtk.js scene test is deselected because this local environment has no trame (it fails without this change too). The pre-commit hooks pass.