Dialogs: sliders step their field's number with the arrow keys

A dialog's slider could take the keyboard focus from Tab, but it
responded only to the pointer, so an arrow key moved the focus to the
nearest widget in the arrow's direction and a keyboard user could not
move the slider. Screen readers could not step it either.

These dialog sliders, each beside a number field, now respond to the
keyboard. A click or Tab gives a slider the focus, which an outline
shows. Then the Right and Up arrows step the field's number up to the
next whole number and the Left and Down arrows step it down to the
next whole number. Shift with an arrow, Page Up and Page Down step it
by ten, and Home and End set the slider's first and last value. No
step goes past either end, and a number past the slider's range steps
from its nearer end. Screen readers' increment and decrement actions
step the slider as the arrows do. A step sets the field's number in
the field's own unit, so a Move slider in a millimeter document steps
one millimeter. When the field holds a number past the slider's range,
Home, End or a drag to that end of a form slider now sets the end's
number; before, such a drag left the field's number as it was.

The form sliders get this through the new color_slider_stepped, next
to color_slider_valued, and so do the Pencil, Paintbrush and Smooth
Fidelity, the Perspective Grid's Opacity, the 3D dialogs' sliders and
the flattener's balance, which the Flattener Preview panel also shows.
The Color panel, Swatch Options, the Color Picker's channel slider,
the Navigator and the Character panel keep their sliders as they were.

Docs: docs/target-app-parity.md.

docs/target-app-parity.md

@@ -479,6 +479,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
   - With anchor points selected with Direct Selection, the contextual task bar offers Remove Anchor Points, Connect (`path.join`), Cut Path, Corner and Smooth (`path.convertAnchors` with `to` set to `corner` or `smooth`), the commands of the Control bar's anchor buttons. As in Illustrator ("Remove selected anchor points, Connect selected end points, Cut path at selected anchor points, Convert selected anchor points to corner, and Convert selected anchor points to smooth": [Contextual Task Bar overview](https://helpx.adobe.com/illustrator/desktop/get-started/learn-the-basics/contextual-task-bar-overview.html)). Not done: Simplify, which the bar offers only while no anchors are selected, and the bar's Smooth item that adjusts a path's curvature, a command the app doesn't have (Object › Path › Smooth).
   - Tool flyouts tear off: dragging or clicking the tear-off bar on a flyout's right side (or releasing the long press over it) floats its group as a strip of tool buttons, moved by the same bar and put back in the toolbar by the × at its top (`window.floatTools`). While a group floats, the presses that would open its flyout raise the strip instead. Floating strips are kept with the preferences and in user workspaces. Not done: turning a strip vertical, docking it.
   - Screen readers get a name for the Tools panel's tool buttons, flyout rows, floating strips and tear-off bars, and for the icon buttons, text buttons, toggles, checkboxes, radio buttons, tabs, chips, sliders, angle dials and combo boxes that panels and dialogs draw with `crates/ui-egui/src/widgets.rs`: the text, label or tool tip each one shows, or a name of its own where it shows none (the angle dial, the spinner, the preset arrow, the color wheel and the Reference Point's squares). The tool buttons, toggles, checkboxes, radio buttons, tabs and the Reference Point's nine squares also report whether they are on, an icon button that is lit, such as the chosen Align To button, reports that it is pressed, and the angle dial reports its angle. The status bar's First, Previous, Next and Last Artboard buttons have tool tips.
+  - These dialog sliders, each beside a number field, respond to the keyboard: those of the effect, tool option, brush and transform dialogs, the Perspective Grid, the 3D dialogs, and the flattener options, which the Flattener Preview panel also shows. A click, or Tab, gives a slider the keyboard focus, and an outline shows it. Then → and ↑ step the field's number up to the next whole number and ← and ↓ step it down to the next whole number, Shift with an arrow and Page Up and Page Down step it by ten, and Home and End set the slider's first and last value; no step goes past either end. Screen readers' increment and decrement actions step the slider as → and ← do. The Color panel's, Swatch Options', the Navigator's and the Character panel's sliders, and the Color Picker's channel slider, respond only to the pointer. The keys follow the WAI-ARIA slider pattern (<https://www.w3.org/WAI/ARIA/apg/patterns/slider/>); the reference app's help does not describe keys on a dialog's slider, so the step sizes are VectorCraft's own.
   - Screen readers get the value of the color and channel sliders that panels and dialogs draw (the Color panel, the Color Picker, Swatch Options, the Navigator's Zoom, the Character panel's variable font axes, the Perspective Grid and flattener options, and the sliders of the effect, tool option and transform dialogs): the number and text of the field beside each one, such as 128 for G or 50% for an opacity, with the slider's range, which extends to include a number past the slider's end. The harmony wheel's Brightness slider, which has no field, reports its brightness in percent.
   - Panels undock (#495): a dock tab, a panel icon or a popped-out panel's title dragged out of the dock floats that panel inside the window, and the strip right of the dock's tabs floats the whole tabbed group. A floating group moves by its title bar, a tab dragged out of it floats on its own, and dropped on another group's title bar or tabs it stacks with that group; dropped on the dock, which lights up under the pointer, its panels go back where they live (the tabbed group or the icon column), as its × and the panel menus' Float Panel / Dock Panel do (`window.panel.float`, `window.panel.dock`). The Tools panel floats by its title bar and docks again on the window's left edge. Floating groups stack into sets: dropped on the upper or lower half of another group (a line shows where), a group joins it right above or below it, and the set's groups stack top to bottom, each showing one panel, moving together by the top group's title bar, whose × docks them all; a group dragged by its tab strip leaves the set (`window.panel.float {below, above}`). A double-click on a tab collapses a floating group to its tabs (`collapsed`). Sets dropped on the left edge of the icon column or of another dock column dock as dock columns (a multi-column dock, `window.panel.dock {column}`), whose groups stack, tab and tear out as a floating set's do, and whose top strip floats the column again. Floating panels, their sets and the Tools panel's position are kept with the preferences and in user workspaces, and clamped into a smaller window. Not done: panels in separate OS windows, docking a panel into the default tabbed group it doesn't belong to, reordering tabs by dragging, collapsing a set or dock column to icons, dock columns right of the default dock, resizing floating panels and the groups of a set.
   - The dock's double arrow collapses Properties, Layers and Libraries to icons at the top of the icon column, popping out like the other icon panels, and expands them again (`window.collapseDock`, kept with the preferences and in saved workspaces). The icon column's own arrow (expanding the icon panels into a full column) is not done.