Brushes: Delete Brush asks first, and Alt-click deletes without asking

The Brushes panel's Delete Brush button and its panel menu item now ask
before deleting the selected brushes, as Swatches and Graphic Styles do.
OK deletes them in one undo step and strokes that used them become plain
strokes; Cancel leaves the document unchanged. Alt-click (Option-click on
macOS) on the button deletes without asking. The brush.delete command
still deletes at once.

docs/target-app-parity.md

@@ -962,6 +962,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
     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).
     In the Brushes panel, Shift-click selects a range of brushes and Ctrl/Cmd-click adds or removes one. Delete Brush and Duplicate Brush then act on all of them (`brush.delete {names}`, `brush.duplicate {names}`).
+    The bottom Delete Brush button and the panel menu ask before deleting; Alt-click (Option-click on macOS) on the button deletes without asking.
     Document Info has a Brushes category after Graphic Styles. It lists the Brushes panel's brushes, or with Selection Only it lists the brushes that the selected objects' strokes use. The Document Info report and Package's report list them too (`document.info {category: "brushes"}`, `brushNames`).
     Document Info has a Repeat Objects category after Embedded Images. It lists each radial, grid and mirror Repeat object by name with its kind and the number of instances it makes. For a grid it gives the rows and columns instead, followed by the number of instances it draws when that is fewer. With Selection Only it lists the Repeat objects in the selection. The Document Info report and Package's report list them too (`document.info {category: "repeats"}`, `repeatObjects`).
     The Brushes panel menu's Select All Unused selects the brushes that are not used in the document, as the [Brushes](https://web.archive.org/web/20241230205225/https://helpx.adobe.com/illustrator/using/brushes.html) page (archived copy) describes. A brush counts as used when a stroke paints with it in the art (opacity masks included), in symbols, graph designs, pattern tiles or graphic styles, or in the art of a used brush (`brush.unused`).