Brushes: Art Brush Options' Width can follow the pen pressure

Art Brush Options' Width had no pen tablet option, so an art brush
drew at one width whatever the pressure. The Width row now has a
Fixed or Pressure menu and a second width. With Pressure the art is
Width wide at the lightest pen pressure and the second width at the
heaviest, following the pressure the Paintbrush recorded along the
stroke, and half way between the two on a stroke drawn without
pressure. brush.options takes width_mode and width_max; a brush
without them is saved as before.

docs/target-app-parity.md

@@ -841,6 +841,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
   - **Brushes (part of #852):** double-clicking a brush in the Brushes panel (or the panel menu's Brush Options…) opens Calligraphic Brush Options (Name; Angle, Roundness and Size, each Fixed, Random or Pressure with its Variation; live previews of the nib and of a light–heavy–light stroke) or Bristle Brush Options (Shape and the bristle sliders); OK edits the brush and every stroke painted with it. The Paintbrush records the pen pressure along each stroke (kept with the path, saved, and joined when a stroke continues a path); a Calligraphic value on Pressure goes from value − variation at the lightest pressure to value + variation at the heaviest, on the canvas and in SVG, PDF and the other exports. Brushes saved before keep their look. Scatter Brush Options (Size, Spacing, Scatter and Rotation, each Fixed, Random between two values or Pressure from the first at the lightest pressure to the second at the heaviest; Rotation relative to Page or Path), Art Brush Options (Width, Scale Proportionately, Stretch to Fit Stroke Length or Stretch Between Guides with the guides' start and end, the four Directions, Flip Along and Across) and Pattern Brush Options (Scale, Spacing, the five tiles shown, Flip Along and Across, Stretch to fit, Add space to fit or Approximate path) each have Colorization (None, Tints, Tints and Shades, Hue Shift with its Key Color picked from the art's colours) and a live stroke preview; values from files and commands are clamped to their ranges. Still to come: the Art brush's Overlap, auto-generated Pattern brush corners, new tile art picked in Pattern Brush Options, Stylus Wheel, Tilt, Bearing and Rotation, and pen pressure on macOS.
     Type in a scatter brush's art takes the brush's colorization as its paths do: Tints, Tints and Shades and Hue Shift recolor the characters' solid fills and strokes in each copy the brush draws, and Brush Options' Key Color list, whose first color is Hue Shift's default key, counts the characters' fills and their strokes wider than 0. The Brushes page says that a scatter, art or pattern brush takes its colors from the current stroke color and the brush's colorization method (<https://web.archive.org/web/20241230205225/https://helpx.adobe.com/illustrator/using/brushes.html>) and does not mention type there; recoloring the characters' paints like the art's fills and strokes is VectorCraft's own rule. The Symbol Stainer mixes the characters' strokes of type toward the stain as it mixes their fills. Evidence: `tests::scatter_colorization_recolors_type`, `tests::art_colors_list_the_most_used_first`, `tests::stain_mixes_type_strokes_too` (vectorcraft-brush).
     Art Brush Options' Width (1% to 1000%) and Pattern Brush Options' Scale (1% to 10000%) each have a slider beside the field, over the same range in whole percents; screen readers name the slider as its row and read the field's value.
+    Art Brush Options' Width has a Fixed or Pressure menu, from the pen tablet options of the Width pop-up menu on the [Brushes](https://web.archive.org/web/20241230205225/https://helpx.adobe.com/illustrator/using/brushes.html) page (archived copy). With Pressure, the art is Width wide at the lightest pen pressure and the second width at the heaviest, following the pressure the Paintbrush recorded along the stroke, and half way between them on a stroke without recorded pressure (`brush.options` `width_mode` and `width_max`); a stroke inside the art takes the width at the middle of its object. The page does not say how the pressure maps to the width; taking the Scatter brush's minimum and maximum rule and the stroke rule are VectorCraft's own. Stylus Wheel, Tilt, Bearing and Rotation are not built.
     Scatter, Art and Pattern Brush Options have the Tips button for Colorization that the [Brushes](https://web.archive.org/web/20241230205225/https://helpx.adobe.com/illustrator/using/brushes.html) page (archived copy) describes, beside Method. It opens Colorization Tips, which shows each method's name and what it does beside a sample of the brush, as OK would make it, painted with that method on a blue stroke; Hue Shift's sample uses the dialog's Key Color. `ui.dialog.set {field: "tips", value: true}` opens it. Screen readers name the button, the popover and each sample.
     Hue Shift set by `brush.new` or `brush.options` without a key color takes the color the art's fills and strokes use most, as Brush Options already offers it, following the Brushes page, where the key color defaults to the color that stands out most in the art (<https://web.archive.org/web/20241230205225/https://helpx.adobe.com/illustrator/using/brushes.html>). The page does not state how prominence is measured. Counting the fills and strokes that use each solid color, with the first one found winning a tie and black for art with no solid color, is VectorCraft's own rule. A `brush.options` edit that gives only the method keeps the brush's key color, and one that gives only an art brush's start guide keeps its end guide. Evidence: `tests_brushsym::hue_shift_without_a_key_takes_the_most_used_art_color`, `tests_brushsym::art_brush_options_keep_the_scale_fields_an_edit_leaves_out`.
     Bristle Brush Options and `brush.new` take a bristle brush name of at most 31 characters, as the Brushes page states for the Bristle Brush Options. The Name field stops taking characters at 31, and `brush.new` and `brush.options` refuse a longer new name with an error that names the limit. The page does not say how the limit counts characters or what happens to longer names already saved; counting characters and the number added to a taken name, keeping longer saved names, and leaving `brush.duplicate` unlimited are VectorCraft's own rules. A bristle brush that a document or a saved file already holds under a longer name keeps it, and the other brush types take a name of any length. Test: `bristle_brush_names_have_at_most_31_characters` in `tests_brushsym.rs`. Public reference: [Brushes](https://web.archive.org/web/20241230205225/https://helpx.adobe.com/illustrator/using/brushes.html) (archived copy).