[max-models]

Adds a file format for interactive figures on the max-models docs sites, and the writer for it, so a figure is saved once and embedded on any page without plotly.js in the file.

maxplotlib.web

• web_figure(fig, title=, alt=, aspect=, white_plot=) turns any Plotly figure (or its JSON as a dict) into the format; write_web_figure(fig, path, ...) writes it.
• The format is Plotly JSON (data, layout, frames) with no look of its own: no template, no fixed size, and transparent backgrounds. The axes get automargin so the tick labels fit at any size.
• Data palette: a trace whose colour comes from a default cycle (Matplotlib C0–C9, Plotly's colorway, Okabe–Ito) gets trace.meta.color_slot. The page can then give it the theme's colour in that position. Traces without a colour follow layout.colorway, black traces take the text colour, and other explicit colours and colormaps stay.
• Hints in layout.meta.web: version, title, alt, aspect, white_plot (on by default for figures converted from Matplotlib).

Canvas

• canvas.to_json(title=, alt=, white_plot=) returns the web figure; canvas.savefig("x.plotly.json", backend="plotly") writes it. The aspect ratio comes from the canvas size.

Docs

• A "Web figures" guide: saving, embedding with the theme's PlotlyFigure, and the format's specification. The theme reads the palette slots and hints from PlotlyFigure: a data palette and web figure hints starlight-theme#53 on. Before that, it already loads plotly.js on demand and recolours text, grid and backgrounds.

Checks

• 9 new tests; all 340 tests pass. The pre-commit hooks pass, and the site builds with the link check.
• Checked in the theme's example site: a saved canvas takes the palette in light and dark mode.