Forms, a11y · Checkbox, Switch, Radio, RadioGroup, Slider and Select name their role, label and state (#3)

A slider's thumb reports its value, the bounds it can reach and its orientation; a select's trigger its value, placeholder and whether its list is open; a disabled control says so in its description.

AGENTS.md

@@ -92,6 +92,7 @@ Ely GPUI Component. A component library for GPUI, in light and dark.
 - A focus handle a component hands back lives across all its modes: take it with `tab_stop` before any early return, or keyed state drops it while the other mode draws and the handed-back focus lands nowhere. A thread's message keeps one handle folded and open, a Tab stop only while folded (`mail::MailThreadView`).
 - A button whose words and action change with a mode keeps one handle through `Button::focus_handle`, and the component moves focus to it or back to its field as the mode turns (`project::TimeTracker`).
 - A button that waits on the owner shows it with `loading`, which takes no press and stays a Tab stop, so focus holds through the wait; `disabled` is for what is not ready, and a disabled button is no stop (`onboarding::ContactSupport`).
+- A control names its role, label and state on the element that takes focus, for gpui's AccessKit tree (a slider's thumb, a select's trigger). gpui sets no disabled flag, so a disabled control says `state.unavailable` in its description. A thumb bounded by another reports the bounds it can reach. The test platform builds no tree; a live window read through UI Automation, or `Window::debug_a11y_tree_json` while a client is connected, proves it.
 - A help mark, ?, is `primitives::HelpTooltip`; FormLabel's help shows it.
 - A panel that opens from something other than a button goes through `overlays::Popover::with_opener`, which hands the opener its toggle; focus, placement and every way out stay in the popover (`calendar::EventPopover`'s chip). Its content gets a close that hands focus back: a press that ends the panel's work closes it before the owner acts, or a panel whose host leaves the tree strands focus (`onboarding::Hotspot`'s Got it, EventPopover's Edit and Delete).
 - A flow in steps goes through `navigation::Wizard`: Back, Next, Finish and the step's motion live there. Next and Finish share one focus handle, and focus moves to it when Back reaches the first step or a finished step is picked. `onboarding::OnboardingWizard` leaves out its numbered steps (`headless`) and shows a count over a bar in as many parts.